Laser processing of materials - Indian Academy of Sciences

Sādhanā Vol.28,Parts3&4, June/August 2003, pp. 495–562. © Printed in India 

Laser processing of materials 

J DUTTA MAJUMDAR and I MANNA ∗ 

Metallurgical and Materials Engineering Department, Indian Institute of 

Technology, Kharagpur 721 302, India 

e-mail: imanna@metal.iitkgp.ernet.in 

Abstract. Light amplification by stimulated emission of radiation (laser) is a 

coherent and monochromatic beam of electromagnetic radiation that can propagate 

in a straight line with negligible divergence and occur in a wide range of wavelength, 

energy/power and beam-modes/configurations. As a result, lasers find wide 

applications in the mundane to the most sophisticated devices, in commercial to 

purely scientific purposes, and in life-saving as well as life-threatening causes. In 

the present contribution, we provide an overview of the application of lasers for 

material processing. The processes covered are broadly divided into four major 

categories; namely, laser-assisted forming, joining, machining and surface engineering. 

Apart from briefly introducing the fundamentals of these operations, we 

present an updated review of the relevant literature to highlight the recent advances 

and open questions. We begin our discussion with the general applications of lasers, 

fundamentals of laser–matter interaction and classification of laser material processing. 

A major part of the discussion focuses on laser surface engineering that has 

attracted a good deal of attention from the scientific community for its technological 

significance and scientific challenges. In this regard, a special mention is made 

about laser surface vitrification or amorphization that remains a very attractive but 

unaccomplished proposition. 

Keywords. 

Laser processing; laser–matter interaction; laser surface vitrification. 

1. Introduction 

Laser, an acronym for light amplification by stimulated emission of radiation, is essentially a 

coherent, convergent and monochromatic beam of electromagnetic radiation with wavelength 

ranging from ultra-violet to infrared [1]. Laser can deliver very low (∼mW) to extremely high 

(1–100 kW) focused power with a precise spot size/dimension and interaction/pulse time 

(10 −3 to 10 −15 s) on to any kind of substrate through any medium [1–4]. Laser is distinguished 

from other electromagnetic radiation mainly in terms of its coherence, spectral purity and 

ability to propagate in a straight line. As a result, laser has wide applications from very mundane 

(bar code scanner) to most sophisticated (3-dimensional holography), mere commercial 

(audio recording) to purely scientific (spectroscopy), routine (printer) to futuristic (optical 

∗ References in this paper have not been cited or prepared in journal format 

495

496 J Dutta Majumdar and I Manna 

computer), and life saving (surgery) to life threatening (weapons/guide). Laser is useful in 

metrology (length/velocity/ roughness measurement), entertainment (laser light show), medical 

diagnostics and surgery/therapy and optical communication/computation. From printer 

to pointer, surgery to spectroscopy, isotope separation to invisible surveillance and medical 

to material treatment, laser finds a ubiquitous presence mainly for some unique combination 

of properties. These important properties that justify the use of laser in such a wide spectrum 

of applications are (a) spatial and temporal coherence (i.e., phase and amplitude are unique), 

(b) low divergence (parallel to the optical axis), (c) high continuous or pulsed power density, 

and (d) monochromaticity [1–10]. 

Figure 1 presents a brief overview of the application of laser in different fields with diverse 

objective [1]. Though the list is not exhaustive, it serves to show the diversity of application 

of laser. In some applications, the power output is of main concern, e.g. atomic fusion 

and isotope separation. Sometimes, the main reason for using laser lies in its spectral purity 

and coherence (pollution detection, length/velocity measurement, interferometry, etc.), low 

divergence (laser show, pointer/guide, audio-player), or a combination of all of them (communication, 

holography, metrology). Accordingly, a host of lasers capable of delivering a 

wide variety of wavelength, energy, temporal/spectral distribution and efficiency have been 

developed over the last several decades [1]. 

In the present contribution, we would confine ourselves to only laser material processing. 

The intense heat that laser may produce on solid matter enables several types of ultrafast, 

novel and economical processing of material that are distinctly advantageous from the 

quality, productivity and efficiency point of view than that possible with their conventional 

counterparts. We will, at first, review the history of laser and enlist the main types of commercial 

laser used in material processing before introducing the working principle of the most 

Applications of lasers 

Low power applications 

High power applications 

Communication 

Metrology Reprography 

Entertainment 

Military Chemical 

Medical 

Heat 

source 

(LMP) 

Optical 

fibre 

communication, 

telecommunication, 

optical 

data storage 

and computation 

Holography, 

length/velocity 

measurement, 

inspection, 

interferometry, 

alignment 

Printing 

of various 

kinds, 

scanning, 

data 

storage 

Laser 

light 

beam 

show, 

pointers, 

audioacoustic 

recording 

Weapon 

guide, 

atomic 

fusion, 

surveillance 

Spectroscopy, 

isotropeseparation, 

photochemical 

deposition, 

pollution 

control 

Angioplasty, 

tumour 

therapy, 

skin/dental/ 

eye surgery, 

dermatology 

Forming, 

joining 

machining, 

rapid 

prototyping, 

manufacturing, 

coating/ 

deposition, 

surface 

engineering 

Figure 1. 

Application spectrum of lasers.

Laser processing of materials 497 

commonly used ones. Before embarking upon reviewing the current status of laser material 

processing, we will discuss the physics of laser–matter interaction and classify the different 

types of laser processing of materials. Finally, we will present a comprehensive update 

of the studies on different types of laser material processing and highlight the scientific and 

technological aspects of importance. In order to confine ourselves to the prescribed limit, we 

have deliberately reviewed the literature published from 1995 onwards. This cut-off, even 

though arbitrary, was unavoidable due to restriction on the length of the paper. However, this 

restriction applies only to the cited literatures but not to discussing the fundamentals of the 

subject. For further details on laser material processing, the textbook by Steen [1] is the most 

comprehensive source of information. 

2. History of laser and its application 

Laser is surely one of the greatest innovations of 20th century. Its continued development has 

been an exciting chapter in the history of science, engineering and technology. As a versatile 

source of pure energy in a highly concentrated form, laser has emerged as an attractive 

tool and research instrument with potential for applications in an extraordinary variety of 

fields. 

The initial foundation of laser theory was laid by Einstein [11]. Subsequently, Kopfermann 

& Ladenburg [12] presented the first experimental confirmation of Einstein’s prediction. 

In 1960, Maiman [13] developed a ruby laser for the first time. This was followed by 

much basic development of lasers from 1962 to 1968. Almost all important types of lasers 

including semiconductor lasers, Nd:YAG lasers, CO 2 gas lasers, dye lasers and other gas 

lasers were invented in this era. After 1968, the existing lasers were designed and fabricated 

with better reliability and durability. By mid 1970s more reliable lasers were made available 

for truly practical applications in the industrial applications such as cutting, welding, 

drilling and marking. During the 1980s and early 1990s the lasers were explored for surface 

related applications such as heat treatment, cladding, alloying, glazing and thin film 

deposition. 

Table 1 summarises commercially available lasers and their main areas of application. 

Depending on the type of laser and wavelength desired, the laser medium is solid, liquid or 

gaseous. Different laser types are commonly named according to the state or the physical 

properties of the active medium. Consequently, we have crystal, glass or semiconductor, solid 

state lasers, liquid lasers, and gas lasers. The latter (gas lasers) can be further subdivided into 

neutral atom lasers, ion lasers, molecular lasers and excimer lasers. The typical commercially 

available lasers for material processing are (a) solid state crystal or glass laser – Nd:YAG, 

Ruby, (b) semiconductor laser – AlGaAs, GaAsSb and GaAlSb lasers, (c) dye or liquid laserssolutions 

of dyes in water/alcohol and other solvents, (d) neutral or atomic gas lasers – He–Ne 

laser, Cu or Au vapour laser, (e) ionized gas lasers or ion lasers – argon (Ar + ) and krypton 

(Kr + ) ion lasers, (f) molecular gas lasers – CO 2 or CO laser, and (g) excimer laser – XeCl, 

KrF, etc. Wavelengths of presently available lasers cover the entire spectral range from the 

far-infrared to the soft X-ray. 

3. Generation of laser 

Laser is a coherent and amplified beam of electromagnetic radiation or light. The key element 

in making a practical laser is the light amplification achieved by stimulated emission due to the


Table 1. Commercially available lasers and their industrial applications. 

Year of Commercialised 

Laser discovery since Application 

Ruby 1960 1963 Metrology, medical applications, inorganic 

material processing 

Nd-Glass 1961 1968 Length and velocity measurement 

Diode 1962 1965 Semiconductor processing, biomedical 

applications, welding 

He–Ne 1962 Light-pointers, length/velocity measurement, 

alignment devices 

Carbon dioxide 1964 1966 Material processing-cutting/joining, 

atomic fusion 

Nd-YAG 1964 1966 Material processing, joining, analytical 

technique 

Argon ion 1964 1966 Powerful light, medical applications 

Dye 1966 1969 Pollution detection, isotope separation 

Copper 1966 1989 Isotope separation 

Excimer 1975 1976 Medical application, material processing, 

colouring 

incident photons of high energy. A laser comprises three principal components, namely, the 

gain medium (or resonator), means of exciting the gain medium into its amplifying state and 

optical delivery/feed back system. Additional provisions of cooling the mirrors, guiding the 

beam and manipulating the target are also important. The laser medium may be a solid (e.g. 

Nd:YAG or neodymium doped yttrium–aluminum–garnet), liquid (dye) or gas (e.g. CO 2 , He, 

Ne, etc.). For gas and diode lasers, the energy is usually introduced directly by electric-current 

flow, whereas, an intense flash of white light produced by incandescent lamps introduces the 

excitation energy in solid state crystal lasers. The sudden pumping of energy causes the laser 

medium to fluoresce and produce intense monochromatic, unidirectional (parallel/convergent) 

and coherent rays [1,2].Among the commercially available lasers, CO 2 -laser seems one of the 

earliest developed and most popular lasers for material processing because they are electrically 

more efficient (15–20%) and produce higher powers (0·1–50 kW) than other lasers in the 

continuous mode. Despite being less efficient in energy coupling with metals due to longer 

wavelength (10·6 µm), the higher wall plug (∼ 12%) and quantum (∼ 45%) efficiency and 

output power level of CO 2 lasers more than compensate for the poor laser–matter energy 

coupling capability. On the other hand, Nd:YAG and Ruby lasers possess shorter wavelength 

and are more suited to pulsed mode of applications requiring deeper penetration, smaller area 

coverage and precision treatment of materials for specific purposes. 

As illustrated in figure 2a, the CO 2 -laser device consists of three main parts –againor 

laser medium, an optical resonator or cavity with two mirrors, and an energizing or pumping 

source that supplies energy to the gain medium. The chemical species in the gain medium 

determines the wavelength of the optical output. Between the two mirrors, one is a fully 

reflecting and the other a partially reflecting one. From the quantum mechanical principle, 

when an external energy is supplied to an atom, the irradiated atom attains an excited state


Figure 2. Schematic set-up of continuous wave CO 2 laser. (a) The major constituents of the machine, 

(b) initial stage of energy pumping, (c) excitation and de-excitation of the atoms in the medium leading 

to emission of laser and (d) stimulated emission and formation of laser beam. 

(figure 2b). The excited atom spontaneously returns to the ground state (E 1 ) from the higher 

energy state (E 2 ) by emitting the energy difference as a photon of frequency (ν): 

ν = (E 2 − E 1 )/h, (1) 

where, h is the Planck’s constant. This phenomenon is known as spontaneous emission (figure 

2c). A spontaneously emitted photon may in turn excite another atom and stimulate it to 

emit a photon by de-exciting it to a lower energy level. This process is called stimulated emission 

of radiation (figure 2c). The latter is coherent with the stimulating radiation so that the 

wavelength, phase and polarization between the two are identical. A photon interacting with 

an unexcited atom may get absorbed by it and excite it to higher energy state. This situation, 

called ‘population inversion’is created by the pumping source. The photons moving along the 

optic axis interact with a large number of excited atoms, stimulate them and by this process 

get amplified. They are reflected back and forth by the resonator mirrors and pass through 

the excited medium creating more photons. In each round trip, a percentage of these photons 

exit through the partially transmitting mirror as intense laser beam (figure 2d). Finally, the


laser beam is either guided on to the work-piece by using reflecting mirrors or delivered at 

the desired site through optical fibres. 

Figure 3 shows a schematic outline of a solid state neodymium doped yttrium aluminum 

garnet (Nd:YAG). The generation of high average power in Nd–YAG laser systems is accomplished 

by combining several individually pumped laser rods in a single resonator. Pumping 

is performed with arc lamps mounted in a close coupling optical geometry that ensures the 

maximum possible absorption of visible pump radiation by the laser rod. This optimizes both 

pumping efficiency and energy extraction efficiency. Energy pumping selectively energizes 

the Nd ions that subsequently lead to a cascading effect and stimulated emission of light. 

These days, energy pumping is also done with diode lasers of appropriate frequency. Nd:YAG 

laser has 40% quantum efficiency. However, the overall electrical efficiency of YAG lasers is 

low. The ratio of laser output power to electrical input power lies in the range 0·5–3%. The 

major advantages of Nd–YAG laser over CO 2 laser lie in its smaller wavelength (1·06 µm) 

and ability to deliver laser radiation through optical fibers. 

4. Laser–matter interaction in material processing 

The input of energy or energy deposition process from a pulsed/continuous wave laser beam 

into the near-surface regions of a solid involves electronic excitation and de-excitation within 

an extremely short period of time [8–10]. In other words, the laser–matter interaction within 

the near-surface region achieves extreme heating and cooling rates (10 3 –10 10 K/s), while the 

total deposited energy (typically, 0·1–10 J/cm 2 ) is insufficient to affect, in a significant way, 

the temperature of the bulk material. This allows the near-surface region to be processed under 

extreme conditions with little effect on the bulk properties. 

4.1 Lattice heating 

The initial stage in all laser-metal processing applications involves the coupling of laser 

radiation to electrons within the metal. This first occurs by the absorption of photons from the 

Figure 3. Schematic set-up of pulsed solid state neodymium–doped yttrium–aluminum–garnet 

(Nd:YAG) laser.


incident laser beam promoting electrons within the metal to states of higher energy. Electrons 

that have been excited in this manner can divest themselves of their excess energy in a variety of 

ways. For example, if the photon energy is large enough, the excited electrons can be removed 

entirely from the metal. This is the photoelectric effect and usually requires photon energies 

greater than several electron volts. Most laser processing applications, however, utilize lasers 

emitting photons with relatively low energy. The energy of CO 2 laser photons is only 0·12 eV 

while the photons obtained from the Nd:YAG laser have about 1·2 eV of energy. Electrons 

excited by absorption of CO 2 or Nd:YAG laser radiation do not therefore have enough energy 

to be ejected from the metal surface. Such electrons must, nevertheless, lose energy to return to 

an equilibrium state after photon excitation. This occurs when excited electrons are scattered 

by lattice defects like usual non-crystalline regions in a crystal such as dislocations and grain 

boundaries such as the lattice deformation produced by photons. In either case, the overall 

effect is to convert electronic energy derived from the beam of incident photons into heat. It 

is this heat that is useful (indeed necessary) in all surface treatment applications. 

4.2 Energy absorption 

Figure 4 describes the process that is important in electron excitation and excited carrier 

relaxation process involved during laser–matter interaction [8]. Photon interaction with matter 

occurs usually through the excitation of valence and conduction band electrons throughout 

the wavelength band from infrared (10 µm) to ultraviolet (0·2 µm) region. Absorption of 

wavelength between 0·2–10 µm leads to intra-band transition (free electrons only) in metals 

and inter-band transition (valence to conduction) in semiconductors. Conversion of the 

absorbed energy to heat involves (a) excitation of valence and/or conduction band electrons, 

(b) excited electron-phonon interaction within a span of 10 −11 –10 −12 s, (c) electron-electron or 

electron-plasma interaction, and (d) electron-hole recombination within 10 −9 –10 −10 s (Auger 

process). Since free carrier absorption (by conduction band electrons) is the primary route of 

energy absorption in metals, beam energy is almost instantaneously transferred to the lattice 

by electron-phonon interaction. 

Figure 4. Schematic diagram depicting electron excitation and carrier relaxation process in materials 

subjected to intense laser irradiation.


Figure 5. Spatial profile of deposited energy following 

irradiation of solid matter by (a) laser beam 

I(z,t) = I o (t)(1 − R) exp(−xz), (b) electron beam – 

I(z,t) = I o (t)(1 − R E )f E (X/X p ), and (c) ion beam – 

C(z)(Q T / √ [ 

2πR P )exp −((z − R P )/ √ √ ] 

2RP ) . 

4.3 Spatial distribution of deposited energy 

The spatial profile of deposited energy from laser beam is illustrated in figure 5a. For laser 

irradiation, the beam intensity I at a depth z for the normally incident beam of initial intensity 

I o (in W/m 2 ) is given by [8] 

I(z,t) = I o (t)(1 − R) exp(−αz), (2) 

where, I o is the incident intensity, t is time, R and α are the reflectivity and absorption coefficients, 

respectively. Since α is very high (∼ 10 6 cm −1 ) for metals, light is totally absorbed 

within a depth of 100–200 Å. The efficiency of optical coupling is determined by the reflectivity 

(R). R for metals is relatively low at short wavelengths, rises abruptly at a critical 

wavelength (related to the plasma frequency of the free electron plasma), and then remains 

very high at long wavelength [8]. 

For comparison, the deposited energy profile from the other two important directed-energysources, 

namely electron and ion beams, are also shown in figure 5b and figure 5c, respectively. 

The energy deposition profile for electron beam irradiation of matter is given by a gaussian 

function, 

I(z,t) = I o (t)(1 − R E )f E (x/x P ), (3) 

where, R E is the reflectivity for e-beam, x P is the distance (x) that coincides with the peak 

intensity and f E (x/x P ) is the spatial energy deposition profile. The deposition profile depends 

on the energy loss hence on incident energy and atomic number. Thus, electron beam is 

more suited to deep penetration welding than surface engineering applications. Similarly, the 

concentration of the implanted species in ion beam irradiation does not coincide with the top 

surface but lies underneath the surface (4):


C(z) = [ Q T /(2π) 1/2 ] { [ ( ) ]} 

z − 

2 

RP 

R P exp − √ , (4) 

2RP 

Here, C(z) is the concentration of a given species at a vertical distance z, R p is the projected 

range/distance and Q T is the dose. 

4.4 Heating due to laser irradiation 

Usually, the deposited energy of laser irradiation is converted into heat on a time scale shorter 

than the pulse duration or laser interaction time [8]. The resulting temperature profile depends 

on the deposited energy profile and thermal diffusion rate during laser irradiation. Thermal 

diffusivity (D) is related to thermal conductivity (k) and specific heat (C P ) as follows: 

D = k/(ρC P ), (5) 

where, ρ is the density. The vertical distance (z) over which heat diffuses during the pulse 

duration (t p ) is given by, z = (2Dt p ) 1/2 . Here, z in comparison to α −1 determines the 

temperature profile. The condition of α −1 ≪ z is applicable typically for laser irradiation of 

metals. 

Under the one dimensional heat flow condition, the heat balance equation may be expressed 

as [5]: 

ρc p 

∂T (z, t) 

∂T 

= Q(z, t) + ∂ (z, t) 

k∂T 

∂z ∂z 

where, T and Q are the temperature and power density at a given vertical distance of depth 

(z) and time (t), respectively. Q follows a functional relation with z same as (2). The heat 

balance equation (6) may be solved analytically if the coupling parameters (α and R) and 

materials parameters (ρ,k and c P ) are not temperature and phase dependent. However, phase 

changes are unavoidable except in solid state processing. Thus, the heat balance equation is 

solved by numerical techniques like finite difference/element methods. 

Depending on the temperature profile, the irradiated material may undergo only heating, 

melting or vapourization. For surface melting and subsequent re-solidification, the solid-liquid 

interface initially moves away from and then travels back to the surface with the velocity 

as high as 1–30 m/s. The interface velocity is given by v ∝ (T m − T i ), where T m and T i 

are the melting and interface temperatures, respectively [8]. Further details on mathematical 

modelling of heat transfer in laser material processing may be obtained in several textbooks 

[5,6]. 

(6) 

5. Laser material processing 

The increasing demand of laser in material processing can be attributed to several unique 

advantages of laser namely, high productivity, automation worthiness, non-contact processing, 

elimination of finishing operation, reduced processing cost, improved product quality, 

greater material utilization and minimum heat affected zone [1–10]. Figure 6 shows a general 

classification of the laser material processing techniques. In general, application of laser 

to material processing can be grouped into two major classes, (a) applications requiring limited 

energy/power and causing no significant change of phase or state, and (b) applications


Laser material processing 

Involving change of phase/state 

Involving no phase change 

solid → vapour solid → liquid solid → solid 

• Machining (cutting, drilling etc.) 

• Deposition/coating 

• Laser spectroscopy 

• Laser-assisted purification 

• Joining (welding/brazing) 

• Surface alloying/cladding 

• Rapid prototyping 

• Reclamation 

• Surface hardening 

• Bending or forming 

• Semiconductor annealing 

• Shocking/shot-peening 

Forming Joining Machining Surface engineering 

• Bending/straightening 

• Manufacturing 

• Colouring/deposition 

• Rapid prototyping 

• Welding 

• Brazing 

• Soldering/sintering 

• Repair/reclamation 

• Cutting 

• Drilling 

• Scribing/marking 

• Cleaning 

• Surface alloying/cladding 

• Surface melting/remelting 

• Surface amorphization 

• Surface hardening/shocking 

Figure 6. 

Classification of laser material processing. 

requiring substantial amount of energy to induce the phase transformations. The first category 

includes semiconductor annealing and etching, polymer curing, scribing/marking of integrated 

circuit substrates, etc. The second type of application encompasses cutting, welding, 

fusion, heat treatments, etc. The average power and efficiency of lasers are not that important 

for the former category that involves no change in phase or state. Lasers suitable for this group 

of applications include (but not limited to) excimer lasers (KrF, ArF), ion lasers (Ar + , Kr + ), 

metallic vapour lasers (cadmium, selenium, copper, gold), solid state lasers (Nd–YAG, Nd– 

glass), semiconductor lasers (gallium aluminum arsenide, etc.), and molecular lasers (CO 2 , 

CO, etc.). For the second category, laser power/efficiency and interaction-time are crucial as 

the processes involve single or multiple phase changes within a very short time. Because of 

high-energy requirement, for this class of operations, CO 2 and Nd–YAG lasers are practically 

the only choice. 

The classification based on phase changes or no phase changes is too academic to be of real 

use to the end users. From the true application point of view, laser material processing can be 

broadly divided into four major categories, namely, forming (manufacturing of near net-shape 

or finished products), joining (welding, brazing, etc.), machining (cutting, drilling, etc.) and 

surface engineering (processing confined only to the near-surface region) [1–20]. Figure 6 

presents this classification in the lower half of the figure mentioning a few representative 

examples from each category of application. However, this classification is based on the 

general definition and scope of the processes as understood in conventional practice, but is 

certainly not sacrosanct.


Surface 

amorphization 

Colouring 

Figure 7. Process map (schematic) in terms of laser power density as a function of interaction time 

for different examples of laser material processing. 

The domain for different laser material processing techniques as a function of laser power 

and interaction time is illustrated in figure 7 [1]. The processes are divided into three major 

classes, namely involving only heating (without melting/vapourizing), melting (no vapourizing) 

and vapourizing. Obviously, the laser power density and interaction/pulse time are so 

selected in each process that the material concerned undergoes the desired degree of heating 

and phase transition. It is evident that transformation hardening, bending and magnetic 

domain control which rely on surface heating without surface melting require low power density. 

On the other hand, surface melting, glazing, cladding, welding and cutting that involve 

melting require high power density. Similarly, cutting, drilling and similar machining operations 

remove material as vapour, hence need delivery of a substantially high power density 

within a very short interaction/pulse time. For convenience, a single scalar parameter like 

energy density (power density multiplied by time, J/mm 2 ) is more useful for quantifying 

different laser assisted processes. However, the practice is not advisable as the specific combination 

of power and time (rather than their product) can only achieve the desired thermal 

and material effect. 

In the following sections, we review the individual classes of laser material processing and 

the current status of understanding. 

6. Laser forming 

One of the major goals of material processing is to produce finished products of correct 

design, shape, geometry and dimension. Manufacturing a finished product is seldom a one-


step process. On the contrary, developing a final product involves several primary (procuring, 

winning/extraction, selection, blending), secondary (melting, casting, compaction, sintering) 

and tertiary (machining, polishing, shaping) steps that are all inter-related, complex 

and time/energy/manpower intensive. Laser material processing offers a unique possibility 

of manufacturing finished products directly from the raw materials without any elaborate 

intermediate operation [1–4,15]. A one-step fabrication is most attractive, obviously for the 

tremendous economy in time, cost, material and manpower than that necessary for the usual 

route of fabrication. Though such possibility is not unlimited and rather confined to only a 

few types of materials and operations, nevertheless, direct fabrication of components using 

laser as a non-contact tool is obviously a breakthrough that must be vigorously pursued. 

Among the several types of laser assisted forming or manufacturing processes in vogue, 

the major and successful ones include laser assisted bending, colouring, rapid prototyping, 

fabrication, deposition, and laser reclamation/repairing [1–4]. These processes distinguish 

themselves from other laser material processing methods in their proclaimed objective of 

single-step manufacturing of a finished or semi-finished product than serving to any other 

intermediate processing aim like machining, joining or surface engineering. For brevity, we 

will address all these laser-assisted versions of otherwise conventional manufacturing processes 

as laser forming. 

Table 2 summarizes the major and representative studies carried out in the broad area of 

laser forming in the recent past (1995 onwards) [21–38]. These studies are selected primarily 

to emphasize the variety of possibilities and their status in laser forming of materials. 

6.1 Laser bending 

Laser bending is a newly developed flexible technique capable of modifying the curvature 

of sheet metal by thermal residual stresses without any externally applied mechanical forces 

[21–25]. Laser bending may also serve the purpose of straightening thin sheets by a similar 

laser based non-contact process without mechanical forces. The process assumes significance 

due to the ease and flexibility of non-contact processing, amenability to materials with diverse 

shape/geometry, properties and chemistry, and high precision/productivity. Laser bending 

involves a complex interplay between the thermal profile generated by the laser irradiation 

and physical/thermal properties and dimension of the material/work-piece. The dimensional 

accuracy of parts produced by bending processes is a topical issue. In general, the process is 

influenced by many parameters such as laser parameters (power density and interaction/pulse 

time), material properties (thermal conductivity, coefficient of thermal expansion, etc.) and 

target dimensions (thickness, curvature, etc.). Laser bending of high strength alloys has been 

an important motivation for the increasing interest in laser forming process. However, success 

in laser bending of thick (> 1–2 mm) high strength steel or superalloy sheets is not yet 

achieved. The materials mostly amenable to bending are Al/Ti-alloys and stainless or low 

alloy steels. Apart from metallic sheets, the success of laser bending of semiconductor and 

polymeric sheets are eagerly awaited by the semiconductor and packaging industry. 

Chan & Liang [21] have recently studied the influence of reinforcement volume fraction on 

the thermal expansion behaviour and bending angle of the Al2024 alloy reinforced with 15– 

20% SiC in laser bending. Under comparable processing conditions, a larger bending angle is 

obtained for the composite with 15% reinforcement. The coefficient of thermal expansion of 

the composite seems to follow different functional relationships with temperature in different 

temperature regimes. Thus, it is necessary to consider a varying degree of coefficient of 

thermal expansion of the composite to predict the bending angle as a function of laser power 

or interaction time. In fact, thermal profile across a dissimilar layer following laser irradiation


Table 2. Summary of selected studies on laser forming of materials in the recent past (1995 onwards). 

Process Year Material Laser Scope Results Ref. 

Laser bending 

Bending 2001 Al–2024 and 

15–20% SiCp 

composite 

Pulsed laser 

Study the effect of 

reinforcement on 

bending angle and 

compare the 

predicted results 

The smaller the 

reinforcement, the 

higher the bending 

angle. A single 

model cannot 

account for all 

regimes 

[21] 

Bending 2000 Stainless steel 

sheet 

Pulsed 

Nd:YAG 


process parameters 

on bending angle 

Bending varies 

inversely with 

sheet thickness and 

directly with power 

(below a minimum 

or threshold) 

[22] 

Bending 1999 Stainless steel 

sheet 

Nd:YLF, 

line-shaped 

pulsed 

Finite element 

modelling of 

thermoelastoplastic 

mode of 

deformation 

Reflectivity and 

thermal expansion 

coefficient 

influence the 

bending more than 

any other parameter 

[23] 

Bending 

1998 AlCuMg-and 

α/β–Ti alloy 

CW–CO 2 

and Nd:YAG 


laser parameters 

and analytically 

determine plain 

strain 

Bend rate/angle 

primarily depends 

on temperature and 

decreases with 

pulses due to 

material accumulation 

at bend 

[24] 

Ceramic 

coating 

on metals 

1995 Cr 2 O 3 coating 

on SAF 2205 

steel 

CW–CO 2 

Study the interfacial 

bonding strength 

and compatibility 

between Cr 2 O 3 and 

steel 

Up to 200 µm 

thick Cr 2 O 3 

cladding on steel 

possible by laser 

cladding. Stress at 

the interface is 

very high 

[25] 

Laser manufacturing 


metal 

forming 

for repair 

2001 Superalloy CW–CO 2 Epitaxial laser 

metal forming of 

single crystal 

high-pressure 

high-temperature 

turbine blade 

Microstructural 

maps predicting 

(solidification 

microstructure, 

growth 

morphology, 

composition) 

constructed 

[26] 

(Continued)


Table 2. (Continued). 


Laser metal 

forming 

Laser aided 

thixotropic 

casting 

1999 Ni-based 

superalloy 

Laser rapid prototyping 

Laser rapid 

prototyping 

Composite 

surfacing 


prototyping 


prototyping 

3-D microstrcture 

1996 Hydroxyapatite 

+ AISI 316 

stainless steel 

CW–CO 2 

CW–CO 2 

2001 Pulsed-CO 2 Polyvinyl 

chloride 

2001 He–Cd laser 

(325 nm) 

Stereolithography 

Aluminoslicate 

ceramic 

powder 

2000 CW–CO 2 Cu–Ti–C and 

Cu–Ti–Ni–C 

2000 Q-switched 

Nd:YAG 

Li–niobate 

and K–titanyl 

phosphate 

Utilize electron 

back scattered 

image for 

orientation 

determination of 

laser formed layer 

Develop funtionally 

graded clad of 

ceramic-metal layer 

by laser aided 

thixotropic casting 

Develop thick 

(130 µm) masks for 

electrodeposition of 

double metallic 

layers on 

cylinder/rod 

Stereolithographic 

fabrication of free 

standing ceramic 

shapes 

Develop a TiC 

dispersed surface 

composite layer 

Rapid prototyping 

by second harmonic 

generation (SHG) in 

suspended 

nonlinear crystals 

1997 Pulsed laser Al 2 O 3 Develop photonic 

band gap structures 

by laser rapid 

prototyping 

1995 Pulsed laser Al + Al 2 O 3 Develop 3-D 

microstructure by 

laser driven 

movement 

Close control of 

solidification 

micro-structure and 

orientation 

distribution is 

possible in epitaxially 

grown layer 

Functionally + 

compositionally 

graded layers 

(ceramic to metal) 

developed by laser 

thixo-casting under 

vibration 

Cu on rotating Pt 

disc and NiFe 

criss-cross pattern 

on cylindrical rods 

were deposited by 

pattern transfer 

The green was 

successfully 

developed from 

ceramics and 

sintered at 1600 ◦ C 

Addition of Ni helps 

in better melting 

and wetting of TiC 

in the composite 

SHG, that turns 

liquid photopolymer 

into solid, 

is a function of 

particle size and 

density. SHG is 

useful in rapid 

prototyping and 3-D 

image formation 

3-dimensional 

photonic band gap 

structure was 

created by vapour 

deposition of FCT- 

Al 2 O 3 into rods 

Thermal expansion 

by laser irradiation 

allows 1-step directwrite 

prototyping 

[27] 

[28] 

[29] 

[30] 

[31] 

[32] 

[33] 

[34] 

(Continued)




Laser colouring 

Colouring 

bleaching 

Colouring 

deforming 

2000 Pulsed KrF 

excimer and 

Nd:YAG laser 

2000 Femtosecond 

pulsed laser 

Amorphous 

WO 3 film 

Nano-Ag 

embedded 

glass 

Colouring of laser 

deposited thin WO 3 

film by laser 

irradiation and study 

the mechanism 

Study mechanism 

of colouring by 

laser induced 

deformation of 

nano-Ag particles 

Colouring 1996 Excimer laser Stainless steel Restore colour (and 

avoid 

discolouration) of 


Colouring 

1995 Femtosecond 

red/UV laser 

Copper 

Study the 

photoemission 

process in 

ultra-short pulses 

Brown (photochemical 

activation) 

to purple (photothermal 

oxidation) 

colours obtained by 

colouring/bleaching 

action 

Time frame for 

transient extinction 

dynamics changes 

(along with colour) 

due to surface 

plasmon resonance 

Thermochemical 

reaction between Fe 

and O 2 produce 

different 

oxides/colours 

Three-(red) and 

two-photon (UV) 

emission process 

occur when Cucathode 

is irradiated 

by red/UV pulses 

[35] 

[36] 

[37] 

[38] 

may generate a large residual stress gradient and cause delaminating or cracking of Cr 2 O 3 

ceramic cladding on steel sheet [25]. 

Laser bending is possible only above a threshold heat input. With sufficient thermal input, 

bending angle decreases significantly with increasing material thickness. However, bending 

angle no longer increases with increasing heat input beyond an upper critical value of 

energy input. The decreasing bend rate with increasing irradiation over the same track may be 

attributed to increase in elastic modulus due to the thickening of the material along the bending 

edge [24]. A two-dimensional plane strain numerical analysis to calculate the bending angle 

in pulsed laser irradiation of stainless steel sheet has shown that both optical reflectivity and 

thermal expansion coefficient constitute the most important considerations that influence the 

precision of the predicted bending angle [23]. However, suitable correlation between bending 

dimension and laser parameters would require proper estimation of the effect of relevant 

material properties at high temperature on the laser bending. 

6.2 Laser manufacturing 

Laser manufacturing is a new materials-processing technique that utilizes the highpower 

lasers to induce controlled thermal changes of shape/dimension/geometry, phase 

(solid/liquid/gas) or function (end use) of a given material or component to manufacture a


semi-finished/finished product/component with a unique precision, versatility and novelty 

[26–28]. 

A very recent application of laser forming with a far-reaching industrial significance concerns 

laser deposition and repair of single-crystal high-pressure high-temperature gas turbine 

blades. The process combines the advantages of a near net-shape manufacturing with the close 

control of the solidification microstructure in laser forming. Recently, Gaumann et al [26] 

have demonstrated the feasibility of this proposition by a process called epitaxial laser metal 

forming that may be useful in repair of cracked or worn parts of single crystal turbine blades 

and extending the engine life. Through a careful study of the solidification microstructure 

under different laser forming regimes, the processing windows for generation and repair of 

single crystal superalloy turbines by laser deposition have been proposed. If the deposited or 

cladded layer is polycrystalline, the orientation of the epitaxially grown grains can be determined 

by electron back scattered diffraction and mechanical property of the layer correlated 

with orientation of the grains [27]. 

Functionally graded materials are a recent development in composite materials that consist 

of a continuously graded interface between two or more component phases. Such heterogeneous 

structure or assembly can be developed by vapour deposition, plasma spraying, 

electrophoretic deposition, controlled powder mixing, slip casting, sedimentation forming, 

centrifugal forming, metal infiltration, controlled volatilization and self propagating hightemperature 

synthesis. A laser-based approach may be more versatile than the above mentioned 

routes in developing functionally graded materials [28]. 

6.3 Laser rapid prototyping 

One of the most recent applications of laser in material processing is development of rapid 

prototyping technologies, where, lasers have been coupled with computer controlled positioning 

stages and computer aided engineering design to enable new capability [1,15,29–34]. 

This development implies that manufacturers are no longer constrained to shape metals by 

removal of unwanted material. Instead, components can now be shaped into near-net shape 

parts by addition-building the object in lines or layers one after another. Rapid prototyping 

relies on ‘slicing’ a 3-dimensional computer model to get a series of cross-sections that can 

then be made individually. The major techniques for making the slices are stereolithography, 

selective laser sintering, laminated object manufacturing and fused deposition modelling. 

Figures 8a&bschematically show basic processes involved in stereolithography and 

selected laser sintering processes, respectively. In stereolithography, the solid object is made 

by scanning an ultraviolet (UV) laser beam over the surface of a bath of epoxy resin that 

hardens on exposure to the UV light (figure 8a). Once a layer is complete, the base plate 

moves down a little in the bath, and a new layer of liquid flows in over the top to enable the 

next layer to form on top. The layer building continues until the component is ready in the 

desired dimension. In selective laser sintering, instead of liquid resin, a fluidized powder bed 

or sheet is used that is heated to close to its melting point (figure 8b). The carbon dioxide 

laser beam scans over the powder and heats the grains so that they undergo incipient on skin 

melting and sinter. Subsequently, the base plate moves down slightly, and the next layer of 

powder is spread across the surface by a rotating roller. The process continues until the desired 

shape or object is ready. 

Figures9a&bpresent the scheme of laminated object manufacturing and fused deposition 

process of prototyping, respectively. In laminated object manufacturing, the preform is built 

from the layers by pulling long and thin sheets of pre-glued paper/plastic across the base 

plate and fixing it in place with a heated roller that activates the glue (figure 9a). A computer


Figure 8. 

sintering. 

Schematic set-up for laser rapid prototyping by (a) stereolithography and (b) selected laser 

controlled laser head scans the surface and cuts out the outline of the desired object. As the 

base plate moves down, the whole process starts again. At the end of the build process, the 

little crosshatched columns are broken away to free the object. In fused deposition process, 

the object is made by squeezing a continuous thread of the material through a narrow nozzle 

(heated by laser) that is moved over the base plate (figure 9b). As the thread passes through 

the nozzle, it melts only to harden again immediately as it touches (and sticks to) the layer 

below. For certain shapes, a support structure is needed, and this is provided by a second 

nozzle squeezing out a similar thread, usually of a different colour to make separating the two 

easier. At the end of the build process, the support structure is broken away and discarded, 

freeing the object/model. The models made from wax or plastics in this method are physically 

robust. This new fabrication concept allows construction of complex parts, starting from a 

3D–CAD model without a mould. 

Figure 9. Schematic set-up for laser rapid prototyping by (a) thin laminated object manufacturing 

(for laminates or sheets), and (b) fused laser deposition technique (for solid objects).


Most of these additive processes produce polymeric objects and only recently laser sintering 

of metal powders has been commercially introduced. In a similar laser based additive rapid 

prototyping approach, it is possible to fabricate free form alumino-silicate ceramic parts 

by stereolithography starting from the UV curable pre-ceramic suspension [30]. The final 

components are obtained by pyrolysis of the organic binder and sintering at 1600 ◦ C. 

Laser can be a useful tool for in situ rapid prototyping fabrication of composite components 

like cutting tools, shear blades, etc. Lu et al, [31] have fabricated TiC dispersed Cu–Ti–C 

and Cu–Ni–Ti–C composites by laser scanning of ball milled powder mixtures. It is felt that 

addition of Ni improves the integrity and surface quality of the laser-fabricated parts because 

of improved melting and wettability of Cu with in situ TiC. 

Second harmonic generation using a 1·06 mm Q-switched Nd:YAG beam in powdered 

nonlinear crystals suspended in a photopolymeric solution could be useful in high resolution 

rapid prototyping [32]. Since efficient second harmonic generation occurs for very 

small powder grain size, this technique may provide a way of realizing high resolution three 

dimensional imaging in which the feature size could be only a few microns in dimension. 

Laser rapid prototyping enables fabrication of 3-D solid freeforms by material deposition 

in successive layers made of adjacent beads. One such structure developed in this method 

was a 3-D periodic photonic band-gap structure of aluminum oxide that consisted of layers 

of parallel rods forming a face-centered tetragonal lattice with lattice constants of 66 

and 133 µm [33]. A similar laser-driven direct-write deposition technique (from trimethylaminealane 

and oxygen precursors) was successfully utilized to fabricate a 3-D microstructure 

consisting of aluminum oxide and aluminum [34]. These laser deposited rapid prototype 

ceramic components are useful as micromechanical actuators like microtweezers and 

micro-motors. 

6.4 Laser colouring 

A thin oxide layer on the surface developed by controlled laser irradiation may produce a particular 

colour or luster. Lu & Qiu [35] have investigated laser-assisted colouring/discolouring 

and bleaching of amorphous WO 3 thin film during pulsed laser deposition. The original films 

could be coloured from light brown to purple by a single pulse of KrF excimer laser irradiation 

at 248 nm and subsequently bleached to brown by a single pulse of Neodynium–yttrium– 

aluminum–garnet laser at 1·06 µm in air. It is suggested that colouring is due to polaron 

transition or photochemical activation, while photothermal oxidation is responsible for the 

bleaching process. In stainless steel, a thermochemical reaction between oxygen and stainless 

steel is believed responsible for colouring during excimer laser irradiation in air [37]. With 

increasing laser fluences, the temperature rise in the irradiated area of stainless steel surface 

increases, which enhances oxygen diffusion into the surface and oxidation reaction within 

the irradiated area. Thus, laser irradiation in vacuum is an easy way to avoid discolouration 

of stainless steel. 

In order to identify the electronic process involved in colouring, Seifert et al [36] have 

carried out single-colour femtosecond pump-probe experiments to investigate the time dependence 

of laser-induced ultrafast desorption and deformation processes of silver nanoparticles 

in glass. Muggli et al [38] have reported that a single-colour illumination of a copper 

surface by a red or an ultraviolet femtosecond laser pulse yields a three-photon (red) or a 

two-photon (UV) photoemission process. On the other hand, a multicolour and multiphoton 

process ensues when the red and the UV pulses overlap both in space and in time on the photocathode. 

It is shown that this emission process results from the absorption by an electron 

of one red and one UV photon.


6.5 Summary and future scope 

Laser forming offers a range of direct, contact-less and novel fabrication possibility of finished 

products/components of metallic, ceramic and polymeric origin. Laser bending is attractive 

to automobile and aircraft industry both for the precision and productivity involved in the 

process. Laser bending is routinely applied in tailoring the curvature of aluminum, iron and 

titanium based metals and alloys. The main mechanisms for bending are identified as ‘temperature 

gradient’ and ‘buckling’ mechanisms. However, these macroscopic approaches fail 

to identify or address the microstructural changes that accompany or cause bending. There 

are several questions that remain to be addressed/answered: (a) Does bending necessitate 

local/global melting? (b) Why does thickness increase along the bend edge? (c) Why does 

bending rate decrease with number of pass? (d) What microstructural features (grain size, 

texture etc.) accompany and affect bending? 

Laser colouring is a relatively new technique introduced mainly for aesthetics as an offshoot 

of laser cleaning or marking practices. While colour depends on the thin oxide layer on 

metals, similar effect on polymers and ceramics may need introduction of pigments at the 

surface. However, the feasibility and durability of laser colouring strictly depends on the 

material chemistry and laser parameters (principally, wavelength). The process promises a 

great market if success is achieved in colouring not only metals, but ceramics, polymers 

and semiconductors alike. In this regard, the main open questions concern the mechanism, 

reproducibility and economics of the process. 

Both laser-assisted manufacturing and rapid prototyping are the major laser forming processes 

that have found commercialization in many applications. The main reasons for interest 

in these processes stem from the scope of direct (one-step) manufacturing of round or square 

sections, hollow tubes and more complex geometry with the same machine and identical fixture. 

Solid parts are usually made in layers. Thus, the interfaces of the consecutive layers 

remain the weakest point. Investigations are warranted to predict the stresses generated at 

the edges and corners, surface roughness/contour and compositional distribution in the solid 

object. Development or epitaxial-repair of single crystal superalloy component will be a real 

breakthrough. In this regard, more comprehensive treatments of heat and mass transfer in specific 

applications are warranted to establish the reproducibility of the laser forming processes. 

7. Laser joining 

One of the earliest and most widely practiced applications of laser material processing was 

joining of metallic sheets using a continuous wave laser [1,16,17]. Today, the automobile and 

aerospace industry relies on lasers for a clean and non-contact source of heating and fusion 

for joining of sheets. More than on any other conventional process. Laser joining is applicable 

to inorganic/organic and similar/dissimilar materials with an extremely high precision, 

versatility and productivity that can only be matched by electron beam welding. Moreover, 

laser welding can be done in air, unlike the vacuum processing needed in electron beam welding. 

In comparison to conventional or arc welding, laser welding scores several advantages 

like narrow welds with controlled bead size, faster welding with a higher productivity, less 

distortion, narrow heat affected zone, amenability to welding Al/Mg alloys and dissimilar 

materials, and minimum contamination [39,40]. 

Laser joining encompasses welding, brazing, soldering and even, micro welding, sintering, 

etc. Joining of materials on a commercial basis requires a laser source of a high power level, 

high reliability, easy operation and low cost. Hence, pulsed or continuous wave Nd:YAG or


CO 2 laser (very seldom ruby laser, too) are the commonly used lasers for joining. The main 

process variables in laser welding are laser power, beam diameter, beam configuration, travel 

speed of the work-piece, substrate condition (roughness, temperature), filler type/feed rate, 

alloy composition and thermophysical properties of the work piece. Table 3 presents a ready 

reference for the most recent and representative studies on laser joining of materials (1995 

onwards) [41–70]. These studies are selected primarily to emphasize recent advances and 

outline the outstanding issues in using laser as a tool for joining materials. 

7.1 Laser welding 

Laser welding, because of the sheer volume/proportion of work and advancement over the 

years, constitutes the most important operations among the laser joining processes [1,17,41– 

59]. Figure 10 shows the front view of the schematic set-up for laser welding without a filler 

rod. The focused laser beam is made to irradiate the work piece or joint at the given level 

and speed. A shroud gas protects the weld pool from undue oxidation and provides with 

the required oxygen flow. Laser heating fuses the work piece or plate edges and joins once 

the beam is withdrawn. In case of welding with filler, melting is primarily confined to the 

feeding wire tip while a part of the substrate being irradiated melts to insure a smooth joint. 

In either case, the work piece rather than the beam travels at a rate conducive for welding and 

maintaining a minimum heat affected zone (HAZ). 

There are two fundamental modes of laser welding depending on the beam power/configuration 

and its focus with respect to the work piece: (a) conduction welding and (b) keyhole 

or penetration welding (figures 11 a,b). Conduction limited welding occurs when the beam 

is out of focus and power density is low/insufficient to cause boiling at the given welding 

speed. In deep penetration or keyhole welding, there is sufficient energy/unit length to 

cause evapouration and hence, a hole forms in the melt pool. The ‘keyhole’ behaves like an 

optical black body in that the radiation enters the hole and is subjected to multiple reflections 

before being able to escape. The transition from conduction mode to deep penetration 

mode occurs with increase in laser intensity and duration of laser pulse applied to the work 

piece. 

Welding efficiency can be defined as a power (or energy) transfer coefficient (η) where η 

is the ratio between laser power absorbed by the work piece and incident laser power. η is 

usually very small but can approach unity once a keyhole has been established. The melting 

efficiency or melting ratio (ε) is given by, 

ε = [vdWH m /P ] . 

Equation (7) relates the rate of melting (ε) to incident laser power, P , where v is welding 

speed, d is sheet thickness, W is beam width and H m is the heat content of the metal at 

the melt temperature. The maximum value of ε is 0·48 for penetration welds and 0·37 for 

conduction welds [17]. It is apparent that ε never approaches unity even when η ≃ 1. Both h 

and ε can be enhanced under the keyhole welding condition if the absorption coefficient can be 

increased. This can be accomplished by application of absorbent coating, surface roughening 

or texturing, preheating, tailoring of temporal irradiation profile and/or oxidation/nitriding. 

Among various process parameters, the quality and properties of laser weld depend on laser 

pulse time and power density, laser spot diameter/penetration, melt area, melting ratio and 

material properties like absorptivity, specific heat, density, etc. Wang et al [41] have demonstrated 

the versatility of laser welding by carrying out in-situ weld-alloying and laser beam 

welding to join SiC reinforced 6061Al metal matrix composite with titanium. Microstructural


Table 3. Summary of selected studies on laser joining of materials in the recent past (1995 onwards). 


Laser welding of Al-alloys 

Welding/ 

alloying 

2000 SiC+Al−6061 

composite + 

Ti 

CW–CO 2 

Study feasibility of 

alloying + welding 

and microstructure 

Harmful 

needle-like carbide 

formation is 

avoided. Central 

fusion zone 

consists of TiC, 

Ti 5 Si 3 and Al 3 Ti 

[41] 

Welding 2000 C95800 Ni–Al 

bronze 

Diode 

pumped 

Nd:YAG 

Study the influence 

of laser and process 

parameters 

Effect of welding 

speed on weld quality 

and microstructure 

determined 

[42] 

Welding 1999 6061–T6 Al– 

Mg–Si alloy 

2·5kW 

CW–CO 2 

laser 

Study the 

microstructure and 

property of the HAZ 

A kinetic equation 

for dissolution of 

Mg 2 Si is proposed. 

Welding at high 

speed and energy 

density is preferred 

[43] 

Welding 

1997 AA 1100 Al 

alloy 

Pulsed 

Nd:YAG 

laser 


laser parameters on 

welding 

Conditions for pore 

free, conduction 

pool geometry 

welding were 

defined 

[44] 

Welding 1996 8090 Al–Li 

alloy 

3kW 

CW–CO 2 

laser 

Microstructure and 

mechanical 

characterization of 

weldment 

Fusion zone 

dimension, solute 

evapouration, 

hardness, porosity 

and tensile 

properties were 

evaluated 

[45] 

Welding 

1995 Al–Fe–V–Si 

alloy 

Laser welding of steel 

CW–CO 2 


welding parameters 

on microstructural 

characteristics of 

weld zone 

Si/Fe/V rich Al 4 Fe 

type precipitates 

were detected. A 

Si/Fe rich phase 

formed on 

boundaries 

[46] 

Welding 2001 Ni + Au–Ni 

plated steel 

Pulsed 

Nd:YAG 

Study effect of Ni 

or Au/Ni plating on 

welding products 

Welding 2001 Steel CW-laser Theoretical study 

on influence of 

Marangoni effect in 

welding 

Au/Ni plating does 

not affect the 

strength of welded 

steel sheets 

Temperature 

dependence of 

surface tension 

plays a significant 

role 

[47] 

[48] 

(Continued)




Bright 

welding 

Penetration 

welding 

2000 AISI 304 stainless 

steel 

1999 High carbon 

steel 

Welding 1997 Austenitic 


Welding 1997 Stainless steel 

and titanium 

Spot 

welding 

Microgra-vity 

welding 

1996 Stainless steel, 

Kovar, Gold 

Photolytic 

iodine laser 

CW–CO 2 

and diode 

laser 

CW–CO 2 

CW–CO 2 

Semiconductor 

laser 

1995 Stainless steel 70W 

CW–CO 2 

Laser welding of Ti-alloys 

Welding 2001 TiNi shape 

memory alloys 

Melting 2001 Ti–6Al–4V 

alloy 

CW–CO 2 

Pulsed 

Nd:YAG 

laser 

Study welding 

characteristics of 

new PIL laser 

(1315 nm) 


of laser parameters 

on weld quality 


welding parameters 

on fusion zone 

Study the plasma 

plume 

characteristics in 

welding 

Study the defect 

formation 

mechanism in 

packaging 

Study the welding 

characteristics in 

varying gravity 

Study corrosion, 

mechanical and 

shape memory 

properties of 

weldments 

Study (in-situ 

X-ray) the key hole 

formation and 

correlate melt depth 

with laser power 

Extremely narrow 

welding seam with 

very fine fully 

austenitic 

microstructure is 

produced with 

minimum HAZ 

CO 2 laser welding 

gives crack free, 

stronger and wider 

weld pool 

Microstructure was 

mostly austenitic 

(2–3% ferrite). 

Higher speed and 

lower power 

produce better 

welding 

Plasma plume 

maintains local 

thermal equilibrium 

and reaches 11000 K 

Defect/hole and 

center line cracks 

disappear below a 

given power density 

and air gap, 

respectively 

Significant variation 

of Cr-distribution is 

noted in different 

gravity conditions 

Decrease in M s - 

start temperature 

and ductility, and 

increase in amount 

of B2 phase and 

strength, but no 

change in shape 

memory effect are 

observed 

Keyhole formation 

is time dependent 

and its bottom 

matches with melt 

pool. Melt depth 

depends on power 

density 

[49] 

[50] 

[51] 

[52] 

[53] 

[54] 

[55] 

[56] 

(Continued)




Welding 1997 Ti–6Al–4V CW–CO 2 Study fatigue and 

tensile properties of 

weldments 

Welding 1995 SiC-fibre reinforced 

Ti-alloy 

Laser welding of Mg-alloys 

Key-hole 

welding 

Laser brazing 

Hard soldering 

2001 AZ91 and 

AM50 alloys 

CW–CO 2 

6kW 

CW–CO 2 

laser 

2001 Diamond film High power 

diode laser 

Joining 2001 Ni + Au–Ni 

plated Al, 

kovar, steel 

Brazing, 

cladding 

Pulsed 

Nd:YAG 

1999 Metals High power 

808 nm diode 

laser 

Micropatterning 

1997 Si 3 N 4 ceramic 

sheet 

Laser sintering 

Sintering 2001 AISI 304 stainless 

steel 

Sintering 2000 

Nano zirconia 

particles 

KrF excimer 

laser 

High power 

laser 

High power 

lasers 

Study feasibility 

and mechanical 

property of 

weldment 

Study the feasibility 

and welding 

characteristics 

Micro-thermal 

management of high 

power diode lasers 

Study brazing 

characteristics, 


inter-diffusion products 

Study the capability 

of diode lasers in 

material joining 



micro-patterning 

Mathematical 

modelling to predict 

residual stress 

Study the 

microstructure, 

densification and 

grain growth 

Crack initiates at 

base metal due to 

martensite. Aging 

reduces crack 

growth rate and 

produces mixed 

mode 

Butt and scarf 

joints were 

successful. Scarf 

angle < 12 ◦ 

produced fracture 

Welding 

morphology and 

quality are 

correlated to 

energy/heat input 

Hard soldering on 

chemical vapour 

deposited diamond 

film is possible 

Au/Ni braze 

improves the 

adhesion of Au/Niplating 

on Ni and 

other base metals 

(Al, Kovar, steel) 

Diode lasers are 

useful for brazing 

and cladding of 

metallic thin sheets 

The optimum 

power level for 

smooth surface 

finish is determined 

Energy transfer 

through plume and 

molten metal 

considered, and 

strength of sinter 

joints determined 

Different 

crystalline phases 

evolve during 

fast/slow sintering 

[57] 

[58] 

[59] 

[60] 

[47] 

[61] 

[62] 

[63] 

[64] 

(Continued)




Sintering 2000 

Laser soldering 

Soldering 2001 

BaTiO 3 ceramics 

Sn–Ag and 

Sn–Ag–Cu 

solders 

CW–CO 2 

Excimer laser 

Study the 

micro/domain 

structure of sintered 

BaTiO 3 

Study the creep 

property of laser 

ablated composite 

solders 

Soldering 2000 Copper alloys Diode laser Determine process 

window of laser 

soldering 

Soldering 1997 

Microsoldering 

SMT device 

material 

1996 Automatic 

tape bonding 

device 

Laser tissue soldering 

Tissue 1999 Indocyanine 

soldering green, bovine 

serum albumin 

CW-diode 

laser 

Diode 

pumped 

Nd:YAG 

laser 

808 nm diode 

laser 


process parameters 

Study feasibility of 

laser soldering of 

lead frames 

Study the 

photothermal 

effects in laser 

tissue soldering and 

process 

optimization 

Both 

conventional/nonconventional 

domain structures 

are observed 

Nano-Cu particles 

improve the creep 

properties of solders 

at 25–105 ◦ C 

Temperature 

measured at lased 

spots to compare 

that predicted by 

models 

The laser 

parameters for good 

quality and uniform 

solders are 

determined 

Good soldering is 

obtained by 98 ms 

lasing with 

5·8W/lead power 

LS is effective in 

repairing damaged 

tissues with better 

tensile strength and 

minimum lateral 

thermal damage 

[65] 

[66] 

[67] 

[68] 

[69] 

[70] 

studies show that the detrimental needle-like aluminum carbides are completely eliminated. 

The central fusion weld joint consists of TiC, Ti 5 Si 3 and Al 3 Ti along with some large pores. 

Figure 10. Schematic of laser welding without a filler rod 

(front view) (after [1]). The argon shroud removes heat 

and prevents undue oxidation. The relative position of 

the laser focus determines the quality and configuration 

of the weld.


Figure 11. Schematic view of (a) conduction melt pool (semi-circular), and (b) deep-penetration 

(key hole) welding mode (after [1]). The surface boiling and marangoni effect are more in (a). 

Penetration depths of 17–35 µm were obtained in deep penetration welding of C95800 nickel– 

aluminum–bronze using a 3 kW diode pumped Nd:YAG laser [42]. The softened zone in 

laser welding of 6061-T6 Al-alloy could be 1/7th of that obtained in tungsten inert gas 

welding [43]. 

Both conduction-mode and keyhole-mode welding are possible in aluminum [44]. Weld 

pool shapes in aluminum depend on the mean power density of the laser beam and the laser 

pulse time. The transition from conduction- to keyhole-mode welding occurred in aluminum 

at a power density of about 10 GW/m 2 , compared to about 4 GW/m 2 for stainless steel. In 

both materials, large occluded vapour pores near the root of keyhole-mode welds are common 

at higher power density. The pores are due to hydrogen that can be significantly eliminated 

by surface milling and vacuum annealing [44]. 

Autogenous “bead-on-plate” laser-beam welding of Al-alloys by a 3 kW CO 2 laser under 

Ar or N 2 atmosphere is possible in the range 700 to 1300W power, 1500 to 9000 mm/min scan 

speed and focus located at 1 to 3 mm below the surface, respectively [45]. The effects of using 

different gases were evaluated in terms of weld-line appearance, fusion-zone dimension, solute 

evapouration, microhardness, post-weld tensile properties, as well as porosity distribution. In 

comparison to electron beam welding, laser welding yielded a higher fusion-zone depth/width 

ratio, cooling rate and porosity amount, and a lower solute loss and post-weld tensile strain. 

A similar investigation on microstructural evaluation following autogenous bead-on-plate 

CO 2 laser welding of an Al–8·5Fe–1·2V–1·7Si alloy (in wt. %) on 2 mm thick sheet showed 

that the fusion zone microstructure consisted of faceted precipitates around 10 µm in size, 

embedded in a cellular-dendritic α-Al matrix with a sub-micrometre intercellular phase [46]. 

Detailed electron microscopy showed that the faceted precipitates in the fusion zone have 

the Al m Fe-type crystal structure (m ≈ 4) enriched in Si, Fe and V and a crystalline Fe- and 

Si-rich phase formed at the cell boundaries. 

Compositional variation or segregation and related temperature-dependent coefficient of 

surface tension (Marangoni effect) have significant effect on the quality of the conduction 

or deep-penetration welding geometry [48]. However, Ni and Au/Ni plating has marginal or 

no influence on laser welding of thin steel sheets except raising the hardness of the weld 

[47]. However, laser power, welding speed, defocusing distance and the type of shielding gas 

combinations should be carefully selected so that weld joints having complete penetration, 

minimum fusion zone size and acceptable weld profile are produced [51]. Heat input as a 

function of both laser power and welding speed has almost no effect on both the type of 

microstructure and mechanical properties of welds.


Spectroscopic measurement of laser-induced plasma in welding of stainless steel and titanium 

has shown that the plasma-plume over the keyhole consisted of metal vapours diluted by 

argon shielding gas and reached a maximum temperature of 11,000 K [52]. Cheng et al [53] 

have studied the defect formation mechanisms in spot-welding during semiconductor laser 

packaging and concluded that the dimension of hole-formation depends on the laser power 

density. The hole disappeared as the power density is below 3 × 10 5 W/cm 2 . 

Hsu et al [55] have studied the effect of CO 2 laser welding of binaryTi 50 Ni 50 andTi 49·5Ni 50·5 

on shape-memory and corrosion characteristics of these alloys. Though martensite start (MS) 

temperature was slightly lowered, no deterioration in shape-memory character of both alloys 

was observed. The welded Ti 50 Ni 50 alloy consisted of an increased amount of B2 phase, 

showed higher strength and considerably lower elongation than the base metal. The same 

alloy registered satisfactory performance in potentiodynamic corrosion tests in 1·5MH 2 SO 4 

and 1·5 M HNO 3 solutions. However, a significantly higher corrosion rate and a less stable 

passivity was noted in artificial saliva. On the other hand, the pseudoelastic behaviour of 

the laser weld of Ti 49·5Ni 50·5 alloy (in cyclic deformation) indicated that the stress required 

to form stress-induced martensite (σ m ) and permanent residual strain (ε p ) were higher after 

welding due to more inhomogeneous nature of the weld metal. 

Perret et al [56] have reported a novel X-ray based study to characterize the keyhole formed 

during pulse Nd–YAG laser interaction with a Ti–6Al–4V metallic target. The keyhole formed 

during the laser–matter interaction is not instantaneous but grows linearly with the laser 

irradiation time. The keyhole depth is nearly the same as the melt-zone depth.An approximate 

estimation of melt depth can be made simply from the ratio of the power density for melting 

to that of a reference metal. However, the proper microstructural control in laser welding of 

Ti–6Al–4V is needed as tensile fracture of laser-welded specimens usually occurs in the base 

metal due to the presence of martensite in the narrow fusion zone [57]. 

To fabricate practical engineering structures with composites, a technique for joining the 

metal matrix composites to themselves and to monolithic metals is essential. Hirose et al [58] 

have compared the joining processes (e.g., laser welding, diffusion bonding and transient 

liquid phase bonding) for structural applications of continuous fiber reinforced metal matrix 

composites like continuous SiC fiber reinforced Ti–6Al–4V composites. Both butt and scarf 

joints were successful by laser welding to join the composites to themselves or to Ti–6Al–4V 

plate without degrading the microstructure. Fracture usually occurred in base the metal. 

In laser welding with power density beyond 10 4 W/mm 2 , the formation of plasma cavity, 

commonly referred to as keyholes, leads to deep penetration welds with high aspect ratio. 

Marya & Edwards [59] have identified the major factors controlling the magnesium weld 

morphology in deep penetration welding of AZ 91 and AM 50 alloys by a CO 2 laser. Though 

irregular weld cross-section profiles were consistently observed on each material, bead dimensions 

often varied with the welding variables in contrasting ways. These variations could arise 

from base metal vapourization and characteristic radiation. 

7.2 Laser brazing 

Like welding, laser is widely used in brazing non-ferrous metals/alloys and ceramics/glasses 

[60–62]. Lorenzen et al [60] have demonstrated the possibility of performing hard soldering 

on a chemical vapour deposited diamond film using a new technique based on high-power 

diode laser. Similarly, Schubert et al [61] have shown that a 1·4 kW diode laser (808 nm) and 

a rectangular spot can be very useful in cladding and brazing. 

Biro et al [47] have studied the effects of Ni and Au/Ni plating on laser brazing/welding 

of 200 µm thick aluminum, nickel, Kovar, and cold-rolled steel sheet in the lap-joint config-


uration using a pulsed Nd:YAG laser and a range of weld process conditions. The strength 

of the joint was equivalent to that of the annealed base material. Significant gas porosity was 

observed at the interface between the Al weld pool and the unmelted Ni and Au/Ni plating 

layers. However, porosity did not affect the tensile shear strengths of the Al joints. Au/Ni 

braze increased the strength of the Au/Ni-plated Ni specimens to that of the base material. 

Heitz et al [62] have reported a detailed investigation on KrF excimer-laser ablation and micro 

patterning of Si 3 N 4 to identify the ablation threshold in air. The surface morphology was flat 

or cone-type at 4 J/cm 2 . Typical ablation rates for obtaining smooth surfaces free from pores, 

scratches, and cracks lie between 0·1to0·2µm/pulse. 

7.3 Laser sintering 

Though sintering is not a true bulk joining process, laser assisted sintering involves localized 

incipient fusion and joining of particles spread over a thin section thickness (unlike conventional 

sintering by bulk heating), and hence, is included in the present discussion. Instead of 

direct synthesis of a component, laser irradiation may be useful to develop a desired shape 

with adequate strength by laser assisted sintering of a pre/directly deposited green compact. 

For instance, laser may be useful in sintering/densification of sol-gel processed nano-ZrO 2 

ceramic coatings [64] and pressure-less sintering of ferroelectric domains in barium titanate 

[65]. In sintering of BaTiO 3 , typical herringbone, square net, and watermark morphologies 

are noted. Furthermore, the wedge-shape and overlapped microtwin domains characterize the 

delta-fringes. 

The advances in miniaturization and ever increasing complexity of integrated circuits frequently 

mean an increase in the number of connections to a component with simultaneous 

reduction in pitch. For this emerging smaller contact geometry, micro-laser connection technologies 

are required. The reliability of the connection plays a decisive role. The implementation 

and reproducibility of laser connection technology in microelectronics depend on good 

thermal contact between the two parts and high quality absorption of the material surface used. 

Laser energy can cause local melting due to overheating of the lead because of the low distance 

between lead and bump. This effect influences the reproducibility of the contacts. Even 

the slightest interruption in the thermal contact of the parts can cause non-reproducibility of 

the contacts. Materials with a higher quality of absorption, for example Sn (32%), can be soldered 

with a good level of reproducibility unlike that for gold (4%) or copper (7%) surfaces. 

Due to the low absorption of these materials it is necessary to use a laser with a higher intensity 

or lower wavelength to produce the same energy. In this regard, reproducibility depends 

on irregularity in absorption, laser instability and thermal contact. 

7.4 Laser soldering 

Laser soldering of semiconductor devices needs extreme care and perfection. However, the 

precision of laser soldering is far superior to conventional soldering. For instance, 215 µmwidth 

tape-automated bonding leads could be soldered to with a device with 430 µm centerto-center 

spacing by 98 ms diode-pumped Nd:YAG laser at 5·8W/lead laser power. Like conventional 

soldering, one of the major concerns about laser soldering remains the mechanical 

property, particularly creep resistance at elevated temperatures. Guo et al [66] have studied the 

creep behaviour in Cu and Ag particle-reinforced composite and eutectic Sn–3·5Ag and Sn– 

4·0Ag–0·5Cu non-composite Pb-free solder joints at 25, 65 and 105 ◦ C (representing homologous 

temperatures ranging from 0·61 to 0·78). Creep resistance in composite solder joints 

was significantly improved with Cu particle reinforcements, but not with Ag-reinforcement.


The strain at the onset of tertiary creep for Cu- and Ag-reinforced composite solder joints was 

typically lower compared to non-composite solder joints. The activation energies for creep 

were similar for all the solder materials. Brandner et al [67] have explored soldering of thin 

or narrow Cu-based electrical joints with solid state (1064 nm) and diode (808 nm) lasers to 

identify the mechanism of energy coupling, mode the temperature rise and determine the process 

window. Laser soldering may be more beneficial for secondary flux-less re-flow solder 

bumping than that in a furnace. 

Berkowitz & Walvoord [68] have studied laser soldering of fine pitch printed wiring boards 

using a continuous wave diode laser. The success of laser soldering of such semiconductor 

devices depends on selection of laser, optimization of laser parameters, surface preparation/cleaning, 

lead form configuration, solder selection/ application and soldering process 

control. It may be noted that solder inter-diffusion in die bonding is important for good bonding 

and hence, high power devices must be mounted in the epitaxy-side down configuration 

for good heat transfer in the well-controlled, high yield and void-free die-attach method. 

7.5 Laser tissue soldering 

Laser has now opened new possibilities in joining organic substances including human and 

animal tissues and organs. Laser tissue welding uses laser energy to induce thermal changes 

in the molecular structure of the tissues. In laser tissue welding, low-strength anastomoses 

and thermal damage of tissue are major sources of concerns. On the other hand, laser tissue 

soldering is a bonding technique in which a protein-solder is applied to the tissue surfaces 

to be joined, and laser energy is used to bond the solder to the tissue surfaces. The addition 

of protein solders to augment tissue repair procedures significantly reduces the problems of 

low strength and thermal damage associated with laser tissue welding techniques. McNally 

et al [70] have attempted to determine optimal solder and laser parameters for tissue repair in 

terms of tensile strength, temperature rise and damage and the microscopic nature of the bonds 

formed. An in vitro study was performed using an 808 nm diode laser in conjunction with 

indocyanine green (ICG)-doped albumin protein solders to repair bovine aorta specimens. 

Liquid and solid protein solders prepared from 25% and 60% bovine serum albumin (BSA), 

respectively, were compared. Increasing the BSA concentration from 25% to 60% greatly 

increased the tensile strength of the repairs. A reduction in dye concentration from 25 to 

0|.25 mg/ml was also found to result in an increase in tensile strength. Increasing the laser 

irradiance or surface temperature resulted in an increased severity of histological injury. The 

strongest repairs were produced with an irradiance of 6·4 W/cm 2 using a solid protein solder 

composed of 60% BSA and 0·25 mg/ml ICG with a surface temperature around 85 ± 5 ◦ C and 

thermal gradient of about 15 ◦ C across 150 µm thick solder strips. Post soldering histological 

examination showed negligible evidence of collateral thermal damage to the underlying tissue. 

Microstructural study revealed albumin intertwining within the tissue collagen matrix and 

subsequent fusion with the collagen as the mechanism for laser tissue soldering. Thus, laser 

tissue soldering may be effective for producing repairs with improved tensile strength and 

minimal collateral thermal damage over conventional tissue welding. 


Laser joining is one of the earliest recorded applications of laser material processing. That laser 

can heat a material irrespective of its chemistry, state, bonding or size/geometry, is obviously 

a big advantage in joining a component with another of the same type or not by using laser 

a clean source of heating. Obviously, joining special steels and alloys constitutes the main


demand for laser welding or joining. Apart from routine joining, laser is useful in joining 

dissimilar materials like steel-Al, steel-alumina, polymer-metal and so on. Even, joining of 

tissues and organic substances is now commonplace using laser. The main issues for future 

research concern joining materials with dissimilar physical (melting point, density, diffusivity) 

and mechanical (hardness, strength) properties and bonding (e.g., metallic to covalent). While 

several attempts have been made to develop mathematical models to predict the microstructure 

and width of the joint with similar materials, similar approaches now need to be extended for 

dissimilar materials. In this regard, capability to predict the microstructure and strength of the 

joints would be the core issues. In particular, investigations on the changes in microstructural 

and mechanical properties across the weldment as a function of the selected process/laser 

parameters are needed. Perhaps, using filler rods can be dispensed with if mechanism for 

brittleness of the joints in certain combinations is well understood. Apart from welding, 

attempts must be made to develop useful localized joining methods like sintering and brazing 

to fabricate finished products of greater variety and challenges. For further details about the 

technology and mechanism of laser joining, several recently published text books may be 

consulted [16,17,39]. 

8. Laser machining 

Laser machining refers to controlled removal of material by laser induced heating from the surface 

or bulk of the work piece and includes laser assisted drilling, cutting, cleaning, marking, 

scribing and several other forms of material removal/shaping [1,2,6]. Laser cutting is the most 

common industrial application of laser in material machining. Metals, ceramics, polymers 

and composites may be laser cut or drilled irrespective of the hardness. Processing is easily 

automated for speed and accuracy giving clean edges with minimum heat affected zone. The 

advantages of laser cutting over other techniques are: flexibility and automation-worthiness, 

easy control of depth of cut, cleanliness, non-contact processing, speed, amenability to a 

wide variety of materials (ductile/brittle, conductor/non-conductor, hard/soft), negligible heat 

affected zone, narrow kerf, and so on [1]. 

Table 4 provides a summary of the most recent (1995 onwards) and representative studies 

on laser machining of materials [71–91]. The areas covered under laser machining include 

cutting, drilling, cleaning (or paint stripping), marking and scribing. These processes are 

applicable to very soft polymers to most hard ceramics, normal incidence to oblique/inclined 

irradiation, ultra-thin semiconductor chips to multi-layer metallic films/sheets, and simple/curved 

surfaces to complex geometry. The discussion in this section will highlight the 

scope/versatility of laser machining and emphasize the recent advances and outstanding 

issues in using laser as a non-contact and non-contaminating tool for machining all kinds of 

materials. 

8.1 Laser cutting 

The general arrangement for cutting with a laser is shown in figure 12 (a,b) [1]. The cutting 

is done either using a transmissive (glass) or reflecting (metal mirror) optics. The transmissive 

optics is made of ZnSe, GaAs or CdTe lenses for CO 2 lasers or quartz lenses for YAG 

or excimer lasers. The reflective optics consists of parabolic off-axis mirrors. The main constituents 

for control and monitoring are the lasers with shutter control, beam guidance train, 

focusing optics and a computer controlled translation stage to move the work-piece. The 

shutter is usually a retractable mirror, which blocks and guides the beam into a water-cooled


Table 4. Summary of selected studies on laser machining of materials in the recent past (1995 onwards). 


Laser cutting 

Cutting 2001 Microelectronic 

packaging 

materials 

Modelling 

of laser 

cutting 

Modelling 

of laser 

cutting 

CO 2 and YAG 

(frequency 

multiplied) 

2000 Metals Theoretical 

study – pulsed 

lasers 

Direct pattern 

processing, image 

transfer, contour 

cutting/trimming 

Finite element two 

stage laser cut 

simulation model 

that considers stress 

relief effect and cut 

mechanism 

1998 Metals CW–CO 2 Model laser 

machining as a 

process of reaction 

and energyabsorption 

Cutting 1996 Al–Li + SiC 

metal matrix 

composite 

Modelling, 

cutting 

1995 Mild and 


Pulsed 

Nd:YAG 

CW–CO 2 

Laser drilling 

Drilling 2001 Stainless steel Dual pulse 

Nd:YAG 

Drilling 1999 Al,W,Mo,Ti, 

Cu,FeAg,Au 

Femtosecond 

Ti-sapphire 

Predict empirical 

relation between 


and cut shape/depth 

and HAZ 

Study the laminar 

boundary layer of 

O 2 -assisted LM 

Improve drilling 

quality by two 

synchronized laser 

pulses without 

coaxial gas flow 



drill quality 

LM is useful for 

fine scale direct 

patterning and 

drilling of Cu-clad 

glass fiber 

reinforced epoxy 

laminates 

Cutting is efficient 

in a specific energy 

window. Upper and 

lower corner stress 

depends on 

passivation breakthrough 

due to 

upper corner crack 

Effect of 

absorptivity, mass 

diffusion rate, 

exothermic 

reaction, cutting 

speed, and geometry 

are considered 

Proper choice of 


minimize HAZ, 

improve cut quality 

and efficiency, and 

surface finish 

Cutting includes 

reaction/ evapouration. 

The model 

excludes shockeffects 

Drilling improves 

due to melting by 

the first and 

evaporation and 

recoil process 

during the second 

pulse 

800 nm short pulses 

produce good 

quality holes. A 

model is proposed 

[71] 

[72] 

[73] 

[74] 

[75] 

[76] 

[77] 

(Continued)




Precision 

machining 

Machining/ 

Metallization 

Laser cleaning 

1999 Stainless steel Pulsed laser Create photonic 

band gap metal 

crystals by precision 

laser machining 

1997 Al-nitride Excimer laser Drill high aspect 

ratio 60–300 µm 

via-holes in AlN 

without / with 

metallization 

Cleaning 2000 Stainless 

steel – 3 

compositions 

/history. 

Polymer 

coating, 

removal/ 

cleaning 

Paint 

stripping 

1998 Polymer on 

TiN coated 

Al–Cu alloy 

Pulsed 

Nd:YAG 

Pulsed 

248 nm 

excimer 

(23 ns) and 

Nd:YAG 

(7 ns) lasers. 

1996 Metal surfaces Pulsed 

TEA–CO 2 

laser 

Laser marking 

Marking 2001 Polypropylene Pulsed 

Nd:YAG 

(532 nm) 

Marking 

Cutting 

Study the 

mechanism and 

influence of laser 

parameters on oxide 

removal 


of laser parameters 

in removing 

polymer layer on 

sub-micron TiN 

coated Al–Cu alloy 

Explore complete 

removal of 

resin/paints from 

metal surface 

Use frequency 

doubled YAG laser 

for surface marking 

2000 Thin sheets CO 2 laser Use hollow glass 

waveguide or 

Ag-halide fibers for 

marking 

Etching 1999 Polyethylene 

Polypropylene 

Pulsed 

Nd:YAG 

(532 nm) 

Explore non-contact 

and dry etching of 

polymers 

Cut-off frequency 

lies in 8–18 GHz 

that can be tuned by 

varying the hole 

size and interlayer 

distance 

A bottom substrate 

can considerably 

reduce back-surface 

damage. Shockwave 

analysis and 

resistance reported 

Laser ablation 

expels the oxide 

laser without 

damaging/removing 

the underlying 

metal layer 

Sub-threshold 

ablation by oblique 

irradiation may 

improve cleaning 

efficiency and 

depth. YAG causes 

more surface 

damage 

Surface condition is 

important for 

effective paint 

removal 

Laser marking on 

polymers offers 

more benefit than 

usual processes 

The system is 

capable of cutting, 

heating and marking 

Reaction of laser 

pulses with 

polymers and 

compounds 

investigated 

[78] 

[79] 

[80] 

[81] 

[82] 

[83] 

[84] 

[85] 

(Continued)




Data storage 

1997 Polyethylene 

terephthalate 

Marking 1996 Organometallic 

films 

Cracking 

Marking 

1995 Sodalime/boro– 

silicate glass 

Laser scribing 

Scribing 2001 Textured Si– 

steel 

Scribing 2000 CdTe, ZnO, 

CuInGaSe 2 , 

etc 

Scribing 1998 Soft-magnetic 

ribbon 

Scribing 1997 Al–nitride 

(AlN) 

Infrared laser 

Ar + ion laser 

(514 nm) 

CW–CO 2 

laser 

KrF excimer 

laser 

(248 nm) 

YAG, Cuvapour, 

Excimer 

Excimer and 

YAG lasers 

Excimer laser 

Use laser 

lithography for 

optical storage of 

data 

Explore marking 

with organo-metal 

film on 

ceramics/plastics 

Investigate 

marking/cracking of 

glass by laser 

ablation 

To reduce core-loss 

of transformer steel 

by scribing 

Investigate the 

feasibility and 

mechanism of 

scribing plastics 

To reduce magnetic 

losses of amorphous 

ribbons 

Machining and 

metallization of 

high aspect ratio 

vias in AlN 

Marking is based on 

radiation induced 

crystallization, 

melting and ablation 

Optimum 

power/time, etc. for 

marking by 

photothermal 

deposition outlined 

Marking occurs by 

surface crazing and 

boiling. Residual 

stress causes cracks 

Core loss reduces 

due to domain 

refinement, stress 

relaxation and 

domain wall pinning 

The optimum 

conditions for 

scribing avoiding 

ridge formation are 

defined 

Scribing at 

optimum distances 

reduces core losses 

by 29–50% 

Straight wall 

through hole vias 

(60–300 µm) 

drilled with back up 

substrate 

[86] 

[87] 

[88] 

[89] 

[90] 

[91] 

[79] 

device that measures the input power. During cutting, the mirror rapidly moves out and allows 

the beam to be directed on to the work piece after passing through the beam guide that directs 

the beam to center on a focussing optic. The focussed beam then passes through a nozzle from 

which a coaxial jet flows. The gas jet is needed both to aid the cutting operation and to protect 

the optics from spatter. For cutting processes which rely on melt removal by the gas jet there 

is a problem for the metal optics system. To achieve a gas jet suitable for cutting (> 20 m/s 

and reasonably well focussed) without interposing a transmissive elements, a set of centrally 

directed nozzles or a ring jet can be used. For cutting non-conducting materials like wood, 

carbon and plastics, the focussed beam heats up the surface to boiling point and generates 

a keyhole. The keyhole causes a sudden increase in absorptivity due to multiple reflections 

and the hole deepens quickly [14]. The parameters controlling the laser cutting operation are


Figure 12. General arrangement for laser assisted cutting using (a) transmissive optics, and (b) 

reflective optics (after [1]). Selection of the optics is based on considerations related to beam power 

vis-à-vis thermal stress limit and operational safety. 

beam diameter, laser power, traverse speed, gas composition, material thickness, reflectivity 

and thermo-physical properties. 

Laser cutting does have a diversified application starting from thin sheet metal cutting for 

general purpose equipment, such as household appliances, electrical cabinets, automotive 

components, thick section metal cutting for trucks, buildings, stoves, construction equipment, 

shipbuilding etc. Titanium alloys cut in an inert atmosphere are used in airframe manufacture. 

Aluminum alloys have similar advantages in cutting by using the laser, which has to be 

well turned and of higher power. Cutting of radioactive material is another important area of 

application of laser cutting. Cutting of wood up to 1" thick for the die-board industry, furniture 

industry, puzzle and gift industry, crafts and trophies, etc. Textile cutting examples include 

plastics, rubbers, composites, cloth, ceramics, etc. Hard brittle ceramics such as SiN can be 

cut ten times faster by laser than by diamond saw. 

Laser cutting is equally useful in manufacturing special materials including laser cutting 

of circuit boards, resistance trimming of circuits, functional trimming of circuits and 

microlithography. The growing use of the excimer laser is of current interest. Hole drilling 

thorough circuit boards to join circuits mounted on both sides has advantages. The excimer 

laser can do this without risk of some form of conductive charring. 

The microelectronics industry is moving toward smaller feature sizes so as to improve 

performance and lower cost. Small distances between chips together with the short interconnection 

routes are conducive for faster operation. Laser processing for via generation, direct 

pattern processing, image transfer, contour cutting and trimming now find increasing application 

in microelectronics packaging industry. On the other hand, smaller spaces between conductive 

patterns increases the risk of short circuits (caused by pattern faults, solder bridges, 

migration, etc.), that emphasizes the need to ensure reliability of laser processing. Illyefaalvi- 

Vitez (2001) has attempted utilization of CO 2 and frequency-multiplied Nd:YAG lasers (using 

five wavelengths, i.e. 10600, 1064, 532, 355 and 266 nm) for drilling and direct patterning 

of copper clad glass fiber reinforced epoxy laminates, polyester foils and similar packaging 

structures. Laser processing was combined with through contacting of the generated vias by 

screen printing with polymer thick films, by wet chemical direct plating and by evapouration 

of thin metal layers. The results show promising opportunities for laser processing of metal 

layers and polymeric materials in microelectronics packaging. 

Analysis of laser metal-cut energy process window is important to predict or develop a 

useful strategy for cutting or machining operation for any engineering material (Bernstein


et al 2000). A laser-energy window exists for each cut structure under a specified laser pulse. 

Experimental observations showed that the differences between upper and lower corner stress 

is temporarily dependent on the passivation breakthrough caused by upper corner cracks. In 

a recent model on metal cutting with a gas assisted CO 2 laser, laser cutting is considered 

as a surface reaction and absorption process that needs an adjusting parameter to represent 

the absorptivity for different materials at different incident angles [73]. The computation of 

the mass diffusion rate at the gas/solid boundary of the cutting front includes the exothermic 

heat released during cutting. It is shown that a very small level of impurity in oxygen 

exerts a significant influence on the cutting performance. Earlier, the combined effect of 

chemical reactions taking place between a gas jet and molten metal was considered adopting 

a laminar boundary layer approach [75]. These models seem to work well for cutting 

speed of up to 30 mm/s and all jet velocities up to sonic speed (as the effect of shock is 

ignored). 

Yue & Lau [74] have studied the pulsed laser cutting of an Al–Li/SiC metal matrix composite 

to examine the influences of laser cutting parameters on the quality of the machined 

surface. Proper process control may minimize the heat-affected zone, improve the quality of 

the machined surface and predict the maximum depth-of-cut for the composite. Finally, the 

optimum condition for achieving high cutting efficiency with minimum material damage was 

recommended. 

8.2 Laser drilling 

High power CO 2 lasers or Nd:YAG lasers may also be used to drill holes. Figure 13 shows 

the basic set used in laser drilling, which is not very different from that used for other laser 

machining processes. Laser drilling can be done in both pulse and continuous wave modes 

with suitable laser parameters. The advantage of the laser is that it can drill holes at an angle to 

the surface – fine lock pinholes in Monel metal bolts is an example. Mechanical drilling is slow 

and causes extrusions at both ends of the hole that have to be cleaned. Mechanical punching 

is fast but is limited to holes further than 3 mm diameter. Electro chemical machining is too 

slow at 180 s/hole but does give a neat hole. Electro discharge machining is expensive and 

Figure 13. Basic system hardware for pulse 

laser drilling of thin sheets of metal, semiconductor 

or polymers. The beam focus coincides 

with the surface. Argon shroud may be need to 

avoid/minimize thermal damage.


slow at 58 s/hole. Electron beam drilling is fast at 0·125 s/hole but needs a vacuum chamber 

and is more expensive than aYAG laser processing. In comparison, aYAG laser takes 4 s/hole 

to outsmart all other methods [1]. 

The ability to machine very small features like holes into a metal or polymer sheet/film 

by laser ablation with an unmatched precision, accuracy and speed has opened a very useful 

scope of application of laser material processing in microelectronic industry. For instance, 

holes with a diameter of 300 nm and depth of 52 nm could be drilled in metal films with 

minimum distortion and heat affected zone using 200 fs and 800 nm pulses from a Ti:sapphire 

laser focused to a spot size of 3000 nm. Lehane & Kwok [76] have developed a novel method 

for improving the efficiency of laser drilling using two synchronized free-running laser pulses 

from a tandem-head Nd:YAG laser capable of drilling through 1/8-in-thick stainless-steel 

targets at a standoff distance of 1 m without gas-assist. The combination of a high-energy 

laser pulse for melting with a properly tailored high-intensity laser pulse for liquid expulsion 

results in the efficient drilling of metal targets. The improvement in drilling is attributed to 

the recoil pressure generated by rapid evapouration of the molten material by the second laser 

pulse. Similarly, Zhu et al [77] have carried out a detailed experimental study of drilling submicron 

holes in thin aluminum foils with thickness ranging from 1·5 to50µm, and W, Mo, 

Ti, Cu, Fe, Ag, Au and Pb foils of 25 µm thickness with femtosecond Ti:sapphire laser pulses 

of 800 nm width. The influence of laser parameters and material properties on hole drilling 

processes at sub-micron scale has been examined and a simple model to predict the ablation 

rate for a range of metals has been developed. 

Laser precision machining has been applied to fabricate metallic photonic band-gap crystals 

(consisting of stainless steel plates with a tetragonal lattice of holes and a lattice constant 

of 15 mm) operating in the microwave frequency [78]. Transmission measurements showed 

that the periodic crystals exhibited a cutoff frequency in the 8–18 GHz range allowing no 

propagation below this level. Furthermore, the cutoff frequency could be easily tuned by 

varying the interlayer distance or the filling fraction of the metal. Combinations of plates with 

different hole-diameters create defect modes with relatively sharp and tunable peaks. 

8.3 Laser cleaning 

Removal of small particles or continuous layers from a metal surface can be carried out by 

laser beam using a selected area irradiation at an optimum combination of incident power, 

interaction/pulse time and gas flow rate (that sweeps the dislodged atoms from the surface). 

Figure 14 shows the schematic mechanism of laser cleaning. At the initial stage, a plasma 

plume is formed due to ionization of the atoms vapourized from the surface and blocks the 

beam-surface contact (figure 14a). As the irradiation stops, the temporary compression on 

the surface changes into tension and causes spallation of the oxidized layer (figure 14b). A 

dramatic improvement of cleaning efficiency in terms of area and energy is possible when the 

laser irradiates the work-piece at an oblique or glancing-angle of incidence rather than direct 

or perpendicular irradiation of the surface. Furthermore, substrate damage is greatly reduced 

and probably eliminated at glancing angles. 

Psyllaki & Oltra [80] have investigated the influence of pulsed laser irradiation on the 

removal of the oxide layer from the surface of stainless steels developed by high temperature 

oxidation using a pulsed Nd:YAG laser. At 1·0–2·0 J/cm 2 , irrespective of the composition and 

thickness of the surface layer, the laser irradiation resulted in the expulsion of the oxide layer 

without any material removal from the underlying metal. 

The demand for new wafer cleaning technology after plasma etching increases as the 

industry enters into submicron processes. The success of low-resistance interconnecting high-


Figure 14. Mechanism of laser cleaning of solid surface with a thin oxide or undesirable layer (a) 

Formation of plasma plume due to ionization of the vapour (at very high laser power for a short time), 

and (b) spallation of the oxidized layer when surface residual stress changes from compressive to 

tensile. 

density ultralarge-scale integrated devices depends on the cleanliness of via holes. The side 

wall and bottom polymers resulting from reactive ion etching of via holes can be removed 

by a non-contact dry laser-cleaning technique using pulsed excimer laser irradiation [81]. 

Similarly, laser cleaning is capable of removing the polymers by sub-threshold ablation, even 

at fluences limited by the damage threshold (= 250–280 mJ/cm 2 ) of the underlying Al– 

Cu metal film with titanium nitride (TiN) antireflective coating. Comparing ablation results 

obtained using Nd–YAG laser and excimer laser shows that although the shorter 7 ns Nd– 

YAG laser pulse gives a greater etch thickness than the 23 ns excimer laser pulse, it also tends 

to damage the metal films and the silicon substrates of the via wafers more easily. 

Tsunemi et al [82] have demonstrated that pulsed laser irradiation of oxidized metallic surfaces 

in an electrolytic cell under proper voltage conditions could be a promising new approach 

for effective removal of oxide films. Systematic measurements on simulated corrosion-product 

films by optical reflectance profile and energy dispersive X-ray spectroscopy showed that the 

utilization of a basic electrolyte solution and imposition of a certain cathodic potential prior 

to laser irradiation were essential for a high removal efficiency. This new technique should 

find potential applications in paint removal metallurgy, semiconductor fabrication technology, 

de-contamination of nuclear power plants and mask-less patterning of oxidized surfaces. 

8.4 Laser marking 

Laser ablation can be applied as a means of direct marking on plastics or polymer compounds 

as it is fast, economical and eco-friendly compared to the conventional marking processes [83]. 

Besides embossing a mark/design on a surface for identification or aesthetics, laser marking is 

useful in non-contact and dry etching and printing of polypropylene or polyethylene plastics 

using a 532 nm laser pulse [85]. Similarly, laser irradiation can produce controlled micro 

cracking or fracture of soda lime and borosilicate glasses [88]. 

Beu-Zion et al [84] have designed and constructed a CO 2 laser marking system based on a 

scanned flexible wave guide (silver halide infrared optical fiber or a hollow glass wave guide) 

capable of cutting, heating or marking on various surfaces. Recently, permanent markings of 

various colours on different ceramic and plastic substrates (with orasols dispersed palladium 

acetate organometallic films) have been made by a photothermal deposition process using an 

argon ion laser (514 nm) [87].


Buckley & Roland [86] have developed a thermal method for lithography on polymer films 

based on selective exposure of the films to infrared laser radiation passing through a mask. A 

high contrast between the image and background is essential in optical data storage processes. 

In this regard, the non-linear response of laser/thermal-marking is inherently advantageous. 

Laser irradiation leads to crystallization, melting and ablation of the polymer and produces 

high-resolution images with excellent edge acuity and minimal interference from diffraction. 

The absence of diffraction effects is due to the nonlinear response of the polymer film to the 

radiation. In this technique, the best resolution achievable is limited by the size of the smallest 

features present on the masks used to create the pattern. 

Recently, attempts have been made to develop a dual-laser-writing scheme in which an 

un-modulated short-wavelength read laser augments the writing process effected by a longerwavelength 

laser. Apart from increasing the thermal efficiency of the laser marking process 

itself, the dual-beam-writing scheme may decrease the mark width and the recorded mark 

length variability. Coupled with the increased resolution of the short wavelength read spot, 

these enhancements could improve the performance. 

8.5 Laser scribing 

Laser machining of ceramics is used extensively in the microelectronics industry for scribing 

and via-hole drilling. Scribing is a process for making a groove or line or line of holes either 

fully penetrating, or not, but sufficient to weaken the structure so that it can be mechanically 

broken. The quality, particularly for silicon chips and alumina substrates, it is measured by 

the lack of debris and low heat affected zone. Thus low energy or high power density pulses 

are used to remove the material principally as vapour. 

Scribing involves laser ablation of a groove or row of holes that form perforation lines to 

separate a large substrate into individual circuits. Via machining is generally followed by a 

metallization step to create three-dimensional interconnections in a multilayer circuit board. 

Lumpp & Allen [79] have drilled high aspect ratio, straight walled 60 and 300 µm diameter 

via-holes in 635 µm thick aluminum nitride with or without a metallization layer deposited on 

the via-walls using an excimer laser. Attachment of a second substrate or metal sheet prevents 

damage to the back surface. 

In fabrication of core laminations for motors and transformers, laser scribing may induce 

reduction in hysteresis and eddy current losses. Laser scribing needs a very low power so as 

not to melt the surface. For example, pulsed excimer laser (248 nm, 23 ns) irradiation to scribe 

the grain-oriented M-4 electrical steel has resulted in a 26% core loss reduction due to the 

beneficial thermal stresses that refined the magnetic domains and reduced the eddy current 

losses [89]. Similarly, 50% reduction in core loss in 50 µm thick amorphous alloy ribbon 

has been achieved by laser scribing at 2–5 mm line spacing at the same excitation power 

[91]. The mechanisms responsible for improvement include magnetic domain refinement, 

stress relaxation and inhibition of domain-wall movement. Laser scribing also relieves the 

stresses that are induced in the material during manufacture. The scribe lines increase the 

surface resistivity of the material, resulting in reduced eddy current loss. Laser scribing of 

polycrystalline thin films used for solar cells can eliminate the frequently observed problem 

of ridge formation along the edges of scribe lines in the semiconductor films [90]. 


Laser machining requires controlled-removal of material by vapourizing within a narrow 

dimension with least heating of the surrounding. The process can be extended to cutting,


drilling, scribing, marking or cleaning. In all these processes, material removal without damaging 

the surrounding is a challenge. The recent advances are now based on selecting appropriate 

wavelength, using multiple beams, allow inclined/oblique incidence and material removal 

in stages. A large variety of materials starting from human/animal tissues to diamond can 

be laser machined, however, an appropriate choice of laser power and wavelength is crucial 

for the success of the operation. Due to diversity of materials, geometry and conditions for 

machining, laser machining now often relies on more than one laser for the same operation. 

Use of advanced optics and better control of the sample stage are the other concerns with 

regards to improving the quality of the cut. Future challenges include increasing the capability 

to machine thicker sections, curved surfaces and dissimilar/ heterogeneous materials. 

Similarly assessment of material damage will need more close control and monitoring of 

the microstructural change/damage across the cut. Development of intelligent machines for 

machining diverse materials would require interfacing a vast database with the hardware. 

Thus, continued efforts are needed to model laser machining processes with suitable experimental 

validation of the predicted results. 

9. Laser surface engineering 

Failure of engineering materials due to corrosion, oxidation, friction, fatigue and wear/abrasion 

is most likely to initiate from the surface because: (i) free surface is more prone to environmental 

degradation, and (ii) intensity of externally applied load is often highest at the 

surface [19]. The engineering solution to minimize or eliminate such surface initiated failure 

lies in tailoring the surface composition and/or microstructure of the near surface region 

of a component without affecting the bulk [2,3,18–20]. In this regard, the more commonly 

practiced conventional surface engineering techniques like galvanizing, diffusion coating, 

carburizing, nitriding and flame/induction hardening possess several limitations like high 

time/energy/material consumption, poor precision and flexibility, lack in scope of automation/ 

improvisation and requirement of complex heat treatment schedule. Furthermore, the 

respective thermodynamic and kinetic constraints of restricted solid solubility limit and slow 

solid state diffusivity impose additional limitations of these conventional or near-equilibrium 

processes [18,19]. 

In contrast, the surface engineering methods based on application of electron, ion and laser 

beams are free from many of the limitations of equilibrium surface engineering methods. 

A directed energy electron beam is capable of intense heating and melting the surface of 

the most refractory metals and ceramics [4]. However, the energy deposition profile in the 

irradiated zone under electron beam is gaussian, and hence, the latter is more suitable for 

deep penetration welding or cladding of similar or dissimilar solids (figure 5). Moreover, the 

scope of generation of X-ray by rapid deceleration of high-energy electrons impinging on 

a solid substrate poses additional disadvantage of a possible health hazard. As an alternative, 

ion beam processing offers practically an unlimited choice and flexibility of tailoring 

the surface microstructure and composition with an implant which has otherwise no or very 

restricted solid solubility in a given substrate [8,10]. However, the peak concentration of 

implanted species, like the energy deposition peak in electron beam irradiation, lies underneath 

and does not coincide with the surface (figures 5b,c). Furthermore, the requirements 

of an expensive ionization chamber, beam delivery system and ultra-high vacuum level are 

serious impediments against large-scale commercial exploitation of ion beam assisted surface 

engineering methods. In comparison, laser circumvents majority of the limitations cited 

above with regards to both conventional and electron/ion beam assisted surface engineering


Laser surface engineering 

Involving only microstructural 

modification 

Requiring changes in both 

microstructure and composition 

Hardening 

(LSH) 

Melting 

(LSM) 

Shocking 

(LSS) 

Texturing 

(LST) 

Alloying 

(LSA) 

Cladding 

(LSC) 

Remelting 

(LSR) 

Deposition 

(LSD) 

Figure 15. 

General classification of laser surface engineering. 

methods and offers a unique set of advantages in terms of economy, precision, flexibility 

and novelty of processing and improvement in surface dependent properties concerned [1– 

4]. One of the major advantage of using laser in surface engineering is its exponential energy 

deposition profile vis-à-vis the gaussian profile of that in electron or ion beam irradiation 

(figure 5). 

Among the notable advantages, laser surface engineering (LSE) enables delivery of a controlled 

quantum of energy (1–30 J/cm 2 ) or power (10 4 –10 7 W/cm 2 ) with precise temporal and 

spatial distribution either in short pulses (10 −3 to 10 −12 s) or as a continuous wave (CW). The 

process is characterized by an extremely fast heating/cooling rate (10 4 –10 11 K/s), very high 

thermal gradient (10 6 –10 8 K/m) and ultra-rapid resolidification velocity (1–30 m/s) [18–20]. 

These extreme processing conditions very often develop an exotic microstructure and composition 

in the near surface region with large extension of solid solubility and formation of 

metastable even amorphous phases. 

Figure 15 presents a brief classification of different LSE methods that involve mainly two 

types of processes. The first type is meant for only microstructural modification of the surface 

without any change in composition (hardening, melting, remelting, shocking, texturing and 

annealing), while the other requires both microstructural as well as compositional modification 

of the near-surface region (alloying, cladding etc.). 

Among the various LSE methods, laser surface alloying (LSA) involves melting of a 

deposited layer along with a part of the underlying substrate to form an alloyed zone for 

improvement of wear, corrosion and oxidation resistance. Figure 16a illustrates the scheme 

of LSA with a continuous wave laser. It includes three major parts: a laser source with a beam 

focusing and delivery system, a lasing chamber with controlled atmosphere and a microprocessor 

controlled sweeping stage where the specimen is mounted for lasing. The process 

includes melting, intermixing and rapid solidification of a thin surface layer with pre/codeposited 

alloying elements (figure 16b). The coating material may be pre-deposited by any of 

the conventional means like electro-deposition, plasma spray and physical/chemical vapour 

deposition or may be injected in the form of powder or powder mixture into the melt at the time 

of laser treatment and is termed as co-deposition. In LSA with pre- or co-deposition, a 20–30 

% overlap of the successive melt tracks is intended to ensure microstructural/compositional 

homogeneity of the laser treated surface. The sweeping stage (x-y or x-y-z-θ) allows laser 

irradiation of the intended area of the sample-surface at an appropriate rate and interaction 

time/frequency. The irradiation results into transient melting of the deposit with a part of the 

underlying substrate, rapid mass transfer by diffusion/convection in the melt pool, and ultra-


Figure 16. (a) Schematic hardware set-up for laser surface alloying (LSA), (b) the processes of 

heating/melting, intermixing and solidification in LSA, and microstructure of (c) a typical alloyed zone 

and (d) solid-liquid interface formed in LSA of Cu with Cr [134]. Note that convection dominates 

mass transfer in the alloyed zone (c), and epitaxial growth marks solidification of the melt pool (d). 

fast solidification to form an alloyed zone (figures 16c,d). The depth, chemistry, microstructure 

and associated properties of the alloyed zone depend on the suitable choice of laser/process 

parameters i.e. incident power/energy, beam diameter/profile, interaction time/pulse width, 

pre or co-deposition thickness/ composition and concerned physical properties like reflectivity, 

absorption coefficient, thermal conductivity, melting point and density. 

9.1 LSE of ferrous alloys 

As already mentioned, LSE is mostly utilized to enhance resistance to corrosion, oxidation, 

wear and similar surface degradation of engineering materials. In this section, we will review 

the scope and current status of understanding about LSE of Fe-based or ferrous alloys. A 

summary of the major studies carried out in the recent past in this direction is provided in 

table 5 [92–150].


Table 5. Summary of selected studies on laser surface engineering of materials in the recent past (1995 

onwards). 

System Year LSE Property Scope Results Ref. 

LSE of ferrous alloys 

AISI 316 

stainless 

steel 

AISI 304 

stainless 

steel 

AISI 1040 

(NiCoCrB) 

AISI 316L 

17-4 PH 

stainless 

steel 

G 10380 

steel G 

41400 

martensitic 

AISI 316L 

austenitic 

UNS– 

S31603 

auste-nitic 

stainless 

steel (Co,Ni, 

Mn,Cr,Mo) 

AISI 304 

stainless 

steeel (Si) 

AISI 304 

stainless 

steeel (Mo) 

2001 LSM Corrosion Improve 

intergrannular 

corrosion and 

intergrannular stress 

corrosion 

2001 LSM Pitting Role of LSM in N 2 

or Ar atmosphere in 

enhancing pitting 

corrosion resistance 

2001 LSA Corrosion, 

erosion 

2001 LSAn Fatigue, 

stress corrosion 

2000 LSS Corrosion, 

stress corrosion 

2000 LSA Corrosion, 

erosion 

Improve both 

corrosion and 

erosion by LSA 

Reduce tendency 

for fatigue crack 

growth and stress 

corrosion cracking 

Introduce 

compressive 

residual stress and 

suppress crack 

growth by laser 

assisted shot 

peening 

Improve both 

corrosion and 

cavitation erosion 

corrosion by LSA 

2000 LSA Corrosion Investigate the role 

of Si in improving 


1999 LSA Pitting, erosion 

Attain AISI 316-SS 

equivalent corrosion 

property on 304-SS 

LSM is effective in 

de-sensitization 

Tendency for pitting 

decreases due to 

dissolution of 

nitrogen and 

formation of nitrides 

Corrosion resistance 

improves in mild 

steel but deteriorates 

in stainless steel 

Laser annealing 

produces duplex 

microstructure that 

retards crack growth 

in quasi-cleavage 

fracture 

Corrosion current 

decreases in 

martensitic steel. 

Laser peeling is 

more effective in 

reducing stress 

corrosion 

Resistance to 

erosion improves 

due to dispersed 

ceramic/intermetallic 

phase but pitting 

resistance decreases 

Si turns the matrix 

ferritic and 

segregates. Post 

LSA 

homogenization 

improves corrosion 

resistance 

Mo improves 

pitting and erosion 


of 304-SS 

[92] 

[93] 

[94] 

[95] 

[96] 

[97] 

[98] 

[99] 

(Continued)




AISI 304 

stainless 

steeel (Si) 

AISI 304 

stainless 

steeel (Mo) 

2000 LSA Corrosion Investigate the role 

of Si in improving 


1999 LSA Pitting, 

erosion 

AISI 316L 1998 LSM Pitting corrosion 

ASTM 

S31254 

stain-less 

steel on mild 

steel 

AISI 304 


(Mo, Ta) 

Mild steel 

(Fe,Cr,Si,N) 

Attain AISI 316-SS 

equivalent corrosion 

property on 304-SS 

De-sensitization of 

nitrogen bearing 


1997 LSC Corrosion Improve pitting 


of mild steel by LSC 

1995 LSMLSA Corrosion, 

wear 

Improve corrosion 

and wear resistance 

by LSM or LSA 

1995 LSA Corrosion Utilize LSA to 

improve corrosion 

resistance of mild 

steel 

Oxidation properties 

Steel (Al) 2001 LSA Oxidation Improve oxidation 

resistance by LSA 

with predeposited Al 

Stainless 

steel 

2000 LC Cleaning Remove oxide layers 

by laser assisted 

vapourization 

Si turns the matrix 

ferritic and 

segregates. Post 

LSA homogenization 

improves 


Mo improves 

pitting and erosion 


of 304-SS 

LSM eliminates the 

sensitized zones and 

significantly 

improves pitting 


Pits following LSC 

are smaller in size 

and number 

LSM produces 

δ-ferrite and 

deteriorates corrosion. 

LSA with Mo 

is useful. Not much 

change in wear 

LSA with Fe–Cr– 

Si–N produces a 

fine duplex 


greatly increases 


LSA with Al forms 

several aluminides 

that imparts good 

oxidation resistance 

at 600 ◦ Cupto200h 

Thin oxide layers 

could be removed 

irrespective of 

chemical 

composition and 

history in a narrow 

energy band 

[98] 

[99] 

[100] 

[101] 

[102] 

[103] 

[104] 

[105] 

(Continued)




Plain carbon 

steel (TiB 2 ) 

AISI 304 

stainless 

steel (Ni–Cr– 

Al–Y and 

ZrO 2 +Y 2 O 3 ) 

Inconel 

800H (Al) 

AISI 316L 

(Fe–Cr–Al– 

Y) 

Wear properties 

S 31603 


(CrB 2 , Cr 3 C 2 , 

SiC,TiC,WC, 

Cr 2 O 3 ) 

Austenitic 


(Nano-Zr) 

Austempered 

ductile iron 

(Cr) 

2000 LSA High 

temperature 

oxidation 

Impart oxidation 

resistance by 

alloying with borides 

1998 LSR Oxidation Improve oxidation 

resistance of 304-SS 

by LSR of prior twin 

(bond + top) coated 

ceramic layers 

1997 LSA Oxidation Improve high 

temperature 


1996 LSC Oxidation Develop thermal 

barrier coating for 

enhanced oxidation 

resistance 

2001 LSA Cavitation 

erosion 

resistance 

2000 LSA Hardness, 

Erosion 

2001 LSALSH Hardness, 

Wear 

Enhance erosion 

resistance by 

developing a ceramic 

dispersed composite 

surface layer 

Develop amorphous 

dispersed composite 

layer to improve 

resistance to erosion 

and wear 

Improve 

adhesive/abrasive 

wear resistance of 

austempered ductile 

iron by LSA or LSH 

Complex oxide 

layers develop on 

steel surface 

exposed to 

600–1000 ◦ C 

following parabolic 

growth rate 

Double remelting 

produces better 


at 1200 ◦ C than 

singleremelted/plasmacoated 

sample 

LSA with Al 

develops an Al-rich 

surface that 

considerably 

improves resistance 

to oxidation up to 

1000 ◦ C 

Rhombohedral- 

Al2O3 and mixed 

Fr + Cr or Ni + Cr 

oxides offer 

enhanced oxidation 

resistance at 

1100–1200 ◦ C 

[106] 

[107] 

[108] 

[109] 

Erosion resistance [110] 

improves 

considerably for all 

carbides and borides 

except Cr 2 O 3 

Hardness and 

wear/erosion 

resistance improves 

significantly due to 

dispersion of Zr-rich 

amorphous phase 

LSH (than LSA) is 

more effective in 

improving wear 

resistance and 

developing 

compressive 

residual stress 

[111] 

[112] 

(Continued)




AISI 1040 

(TiB 2 ) 

Mild steel 

(FeCr–TiC) 

Mild steel 

(Hadfield, 

Fe–Mn–C) 

Pearlitic rail 

steel 

EN31 

bearing steel 

A 7 tool steel 

(Ti) 

Medium 

carbon steel 

(Stellite 6) 

Mild steel 

(WC) 

2000 LSA Tribology, 

Wear 

Develop boride 

coated/dispersed 

surface layer and 

improve wear 

resistance of 

low-carbon steel 

2000 LSR,SHS Wear Utilize SHS and 

LSR to develop a 

TiC dispersed 

surface composite 

1999 LSC Wear Improve hardness, 

wear and bulk 

mechanical 

properties 

1998 LSM Wear Improve mechanical 

and tribological 

properties by LSM 

1997 LSH Wear, 

Friction 

1997 LSA Wear, 

Friction 

Improve wear and 

friction properties by 

LSH 

LSA with excimer 

laser to improve 

wear/friction 

property 

1996 LSC Wear Improve wear 

resistance by LSC 

with stellite coating 

1995 LSR, SHS Hardness Improve hardness 

and wear resistance 

by SHS and LSR 

2. LSE of superalloys and special steels 

Cr–Mo steel 

(Cr) 


temperature 


Martensitic 

steel 

(Ni-alloy) 

1999 LSC Hardness, 

Wear 

Improve wear and 

erosion resistance of 

the base steel 

LSA improves 

resistance to 

adhesive/ abrasive 

wear and reduces 

friction coefficient 

LSR homogenizes 

the microstructure 

and improves wear 

resistance 

LSC improves wear 

resistance and bulk 

mechanical 

properties of the 

clad 

LSM produces a 

thin glazed layer 

with better hardness 

and lower friction 

rate 

Single or double 

glazed LSH tracks 

have better wear and 

friction property 

LSA reduces 

friction coefficient 

but does not affect 

wear properties 

Wear against 

4140/4340 steels 

considerably 

improves due to 

LSC 

LSA enhances 


in 800–1000 ◦ C due 

to Cr 2 O 3 rich scale 

LSC significantly 

increased hardness 

and erosion 

resistance 

[113] 

[114] 

[115] 

[116] 

[117] 

[118] 

[119] 

[120] 

[139] 

[140] 

(Continued)




AISI 304 

stainless 

steel (Ni–Cr– 

Al–Y and 

ZrO 2 +Y 2 O 3 ) 

Si-steel 

(Ni + Cr) 

1998 LSR Oxidation Improve oxidation 

resistance by LSR of 

bond (Ni–Cr–Al–Y) 

and top 

(ZrO 2 + Y 2 O 3 ) coat 

layers 

1998 LSC Oxidation Improve oxidation 

resistance 

Inconel-600 1997 LSM Oxidation Enhance oxidation 

resistance 

Inconel 

800H (Al) 

35 NCD 16 

ferritic steel 

(Cr 3 C 2 /SiC) 


temperature 



oxidation 

3. LSE of non-ferrous alloys 

Al and its alloys – Corrosion properties 

Al–6013 + 

SiC p 

composite 

2014 Al alloy 

(Cr, W, 

Zr–Ni,Ti–Ni) 

Al-alloy 

(Zr 60 Al 15 Ni 25 ) 

Enhance hardness 

and high temperature 


1999 LSM Corrosion Improve pitting and 

general corrosion 

properties by LSM 

1998 LSA Corrosion Study the corrosion 

behaviour ofAl-alloy 

surface after LSA 

1997 LSCLSR Corrosion Surface 

amorphization of Al 

by LSC/LSR to 

improve pitting 


Double remelting 

produces better 


at 1200 ◦ C than 

single-remelted/ 

plasma-coated 

sample 

LSC with high 

Ni–Cr alloy (with 

Si) improves 


at 900 ◦ C. Si content 

is important 

LSM improves 


due to 

microstructural 

homogeneity 

Al-rich surface by 

LSA improves 


up to 1000 ◦ C 

Hardness increased, 

Oxidation 

resistance increased 

due to Si containing 

complex oxides 

Rapid solidification 

of LSM refines the 

surface 


improves the 


Overlapping regions 

are more prone to 

pitting corrosion 

due to segregation 

Amorphous phase 

dispersion (function 

of LSC/LSR 

parameters) 

produces an 

improved corrosion 

resistance 

[107] 

[141] 

[142] 

[108] 

[143] 

[121] 

[122] 

[123] 

(Continued)




2014 Al alloy 

(Cr, W, 

Zr–Ni,Ti–Ni) 

Al-alloy 

(Zr 60 Al 15 Ni 25 ) 

Al–Si (Ni– 

Cr–B–Si) 

Al–SiC 

composite 

1998 LSA Corrosion Study the corrosion 

behaviour ofAl-alloy 

surface after LSA 

1997 LSCLSR Corrosion Surface 

amorphization of Al 

by LSC/LSR to 

improve pitting 


1997 LSR Microstructure 

Study 

microstructural 

evolution in LSR of 

plasma coating on 

Al–Si 

1996 LSM Corrosion Improve corrosion 

resistance by LSM 

with excimer laser 

Al and its alloys – Oxidation properties 

6061 Al alloy 

(Al + TiC) 

2001 LSCLSA Oxidation Improve oxidation 

resistance of Al-alloy 

by LSC/LSA 

Al–Cu–Li 1997 LSM Oxidation Improve oxidation 


assisted surface 

treatment 

AlGaAs and 

AlAs layers 

1996 LSM Oxidation Study wet oxidation 

characteristic of 

buried thin film for 

laser device 

Al and its alloys – Wear properties 

Al + SiC 1999 LSS Hardness, 

wear 

Improve oscillating 

wear property by 

LSS of HVOF 

coated composite 

surface layer 

Al (Al 3 Ti) 1999 LSC Wear Improve wear 

resistance 

Overlapping regions 

are more prone to 

pitting corrosion 

due to segregation 

Amorphous phase 

dispersion (function 

of LSC/LSR 

parameters) 

produces an 

improved corrosion 

resistance 

Ultrafine Ni 3 Al and 

amorphous phase 

with very high 

hardness form by 

LSR 

Microstructural 

homogeneity by 

LSM improves 


TiC + Al composite 

coating improves 


in 200–600 ◦ C 

LSM produces 

adherent scale and 

improves oxidation 

resistance at 450 ◦ C 

Oxidation of 

AlGaAs provides 

suitable apertures 

for surface lasers 

Both hardness and 

wear resistance 

improve by LSS 

due to residual 

compressive stress 

Composite 

intermetallic layer 

on surface enhances 

wear resistance 

[122] 

[123] 

[124] 

[125] 

[126] 

[127] 

[128] 

[129] 

[130] 

(Continued)




Al (AlN) 1998 LSA Hardness Synthesize AlN layer 

by excimer laser irradiation 

in nitrogen 

AlSi 

(Ni–WC) 

1997 LSC Hardness Develop 

carbide/amorphous 

dispersed high 

hardness layer 

Cu and its alloys 

Cu–Cr–Fe 2000 LSR Hardness, 

Wear 

Improve tribological 

properties of powder 

compacts by LSR 

Cu (Cr) 1999 LSA Wear, erosion Improve 

wear/erosion 

resistance of Cu by 

LSA with Cr 

Mg and its alloys 

Mg + SiC 

(Al–Si) 

AZ91D and 

AM60B Mg 

alloys 

AZ91 Mg 

alloy 

Mg − 

ZK60 + SiC 

composite 

2001 LSC Corrosion Improve corrosion 

resistance by LSC 

with Al + Si alloy 

2001 LSM Corrosion Improve corrosion 


1999 LSM Corrosion Enhance corrosion 

resistance 

1997 LSM Corrosion Enhance corrosion 

resistance 

LSA produces 

adherent AlN layer 

on Al with high 

hardness 

Microstructure of 

cladded layer 

consists of 

aluminides + 

amorphous phases 

and has high 

hardness 

LSR improves 

hardness and wear 

resistance and 

reduces friction 

LSA enhances 

resistance to 

adhesive/ abrasive 

wear and erosion 

due to solid solution 

and dispersion 

hardening 

LSC improves 

corrosion 

resistance. LSC 

parameters have 

strong influence 

Uniform and refined 

microstructure 


resistance 

Pulsed excimer laser 

irradiation improves 


Uniform and refined 

microstructure 

produced by 

excimer laser 


resistance 

[131] 

[132] 

[133] 

[134] 

[135] 

[136] 

[137] 

[138] 

(Continued)




Ti and its alloys 

Ti–6Al–4V 

(TiN) 

Ti (Si,Al, 

Si + Al) 

Ti (Si,Al, 

Si + Al) 

Ti–6Al–4V 

(WC,TiC) 

2001 LSA Wear Enhance wear 


with nitrogen 

2000 LSA Wear, friction Improve wear 


1999 LSA Cyclic oxidation 

Improve oxidation 


with Si/Al 

1998 LSA Corrosion Enhance corrosion 

resistance 

Ti (Ti + TiC) 1997 LSA Hardness, 

wear 

Ti and 

Ti–IMI829 

alloy (N 2 ) 

Ti alloy 

(WC, TiC) 


corrosion 

Increase hardness by 

developing a 

composite surface 

layer 

Study microstructure 

and enhance 

hardness and 


1996 LSA Hardness Improve 

wear/galling 

resistance 

TiN forms by LSA 

and improves wear 

resistance that 

occurs in 3 stages 

LSA with Si is 

more effective than 

Al/Si + Al to 

improve wear 

resistance 

LSA with Si/Si +Al 

is more effective 

than Al in enhancing 

cyclic oxidation 

resistance between 

ambience/750oC 

LSA with TiC 

improves, but that 

with WC 

deteriorates 


LSA with TiC 

significantly 

enhances the 

hardness of 

substrate Ti 

LSA enhances 

hardness and 


LSA significantly 

enhances hardness 

[144] 

[145] 

[146] 

[147] 

[148] 

[149] 

[150] 

9.1a Corrosion resistance by LSE: Parvathavarthini et al [92] have attempted to eliminate 

the susceptibility to inter-granular corrosion of the cold worked and sensitized AISI 316 

stainless steel by laser surface melting (LSM). Similarly, Mudali et al [100] have found LSM 

could be useful in de-sensitization of nitrogen bearing 316L stainless steel. The improvement 

by LSM is attributed to complete dissolution of M 23 C 6 type of carbides and suppression of 

re-precipitation due to rapid quenching. LSM in nitrogen atmosphere or of nitrogen bearing 

steel is reported to improve the resistance to pitting corrosion of AISI 304 stainless steel 

[92,93]. In either case, this improvement may arise due to the presence of chromic oxide 

and nitrogen compound on the surface and consequent reconstruction of the passive layer or 

barrier to the electrolyte. Potentiodynamic polarization and ultrasonic vibration studies with


stainless hard-facing NiCoCrB alloy coated mild steel have shown that LSM could markedly 

improve the resistance to corrosion and cavitation erosion-corrosion of the substrate [94]. 

However, attempts to simultaneously improve electrochemical and mechanical properties by 

LSE may be counter-productive as presence of ceramic or intermetallic phases on the surface 

may provide initiation sites for pitting [97]. 

Dutta Majumdar & Manna [99] have investigated the effect of LSM and LSA of plasma 

spray deposited Mo on AISI 304 stainless steel (304-SS). Figure 17a shows the optimum 

conditions (shaded region) for the formation of a homogeneous microstructure and composition 

in this study for improvement in pitting corrosion and mechanical property. Potentiodynamic 

anodic polarization tests of the substrate and laser surface alloyed samples (SS(Mo)) 

in 3·56 wt. % NaCl solution (both in forward and reverse potential) showed that the critical 

potential for pit formation (E PP1 ) and growth (E PP2 ) have significantly (2–3 times) 

improved from 75 mV(SCE) in 304-SS to 550 mV(SCE) (figure 17b). E PP2 has also been 

found to be nobler in as lased specimens than that in 304-SS. The poor pitting resistance 

of the plasma sprayed 304-SS samples (without laser remelting) was probably due to the 

presence of surface defects present in plasma deposited layer. Standard immersion test was 

conducted in a 3·56 wt. % NaCl solution to compare the effect of laser surface alloying on 

Figure 17. (a) Process optimization diagram for selecting the necessary energy density (E) for LSA 

of 304-SS with Mo to achieve the desired composition (X Mo ) and hardness (Hv 

av ) in the alloyed zone 

(shaded region), (b) cyclic potentiodynamic polarization behaviour of 304-SS and SS(Mo) in 3·56 wt. 

% NaCl solution, (c) variation of pit density in 304-SS and SS(Mo) as a function of time (t) in a 

3·56 wt. % NaCl solution, and (d) comparison of material loss per unit area (m) due to erosion as a 

function of t for 304-SS and SS(Mo). For details, please see [99,197].


the pitting corrosion resistance. Figure 17c compares the kinetics of pit formation in terms of 

area fraction of pits determined by standard immersion test in a 3·56 wt.% NaCl solution as 

a function of time (t) between 304-SS and SS(Mo) lased with 1210 MW/m 2 power density 

and 31·7 mm/s scan speed for deposit thickness of 250 µm (corresponding to highest E PP1 ). 

It is evident that both the extent and rate of pitting were significantly reduced in the SS(Mo) 

as compared to that in 304-SS. Furthermore, the process of pitting in 304-SS follows a sigmoidal 

nature marked by a substantially rapid initial stage than that of the later stage with no 

incubation time. In comparison, pits were noticed in SS(Mo) only after 50 h of immersion, 

and the number increases linearly following a much slower kinetics as compared to that in 

304-SS. Continuous circulation of the samples at 750 rpm for 10 to 75 h in a medium containing 

20 wt. % sand in 3·56 wt. % NaCl solution showed significant decrease in the kinetics 

of erosive-corrosion loss in SS(Mo) than that in 304-SS. It was thus concluded that LSA is 

capable of imparting an excellent superficial microhardness and resistance to corrosion and 

erosion-corrosion properties to 304-SS due to Mo both in solid solution and as precipitates. 

In an attempt to develop stainless mild steel by coatingASTM S31254 stainless steel powder 

on plain carbon steel, Anjos et al [101] have noted that the anodic polarization behaviour 

following LSE is similar to the bulk stainless steel even in very aggressive solution. Similar 

efforts of surface alloying of Fe–Cr and Fe–Cr–Si–N layers on plain carbon steel to improve 

corrosion resistance led to development of a Fe–Cr–Si–N surface layer with a fine duplex 

microstructure that showed a higher pitting potential than that of the Fe–Cr layer [103]. The 

passive film resistance increased and passive current density decreased with the increasing 

Cr content. 

9.1b Oxidation resistance by LSE: Oxidation is another serious mode of surface degradation 

that gets aggravated under unabated counter ionic transport of cations and anions at elevated 

temperature. Unlike electrochemical corrosion, oxidation occurs through dry reaction and 

solid state ionic transport through the oxide scale. In the past, several attempts have been 

made to enhance resistance to oxidation by LSA, LSC and similar LSE techniques [104–109]. 

Recently, Pillai et al [104] have obtained enhanced oxidation resistance (at 873 K for up to 

200 h) of plain carbon steel following LSA with Al due to several intermetallic phases like 

Al 13 Fe 4 and Al 2 Fe 2 . Agarwal et al [106] used a composite boride (TiB 2 ) coating to enhance 

oxidation resistance of steel in the range 600–1000 ◦ C. Thermal barrier coatings could also be 

effective for improving oxidation resistance of stainless steel [107]. However, such coatings 

are brittle and hence are applied with a favorable composition gradient like applying a bond 

coat (Ni–22Cr–14Al–1Y) between the topcoat (ZrO 2 + 7·5 wt%Y 2 O 3 ) and stainless steel 

substrate before single or multi pass LSM treatments. Oxidation tests at 1200 ◦ C indicated that 

double pass LSM was more effective to impart a better oxidation resistance [107]. Similar 

attempts of improving oxidation resistance have been made with Incoloy 800 H by LSA 

with Al [108] and by LSC of 316 L stainless steel with Fe–Cr–Al–Y alloy coatings [109]. 

In all these cases, it appears that the degree of enhancement of oxidation resistance by LSE 

primarily depends on stability, adherence, imperviousness and strength of the complex oxide 

scale comprising multiple oxide on top of the base metal. 

Manna et al [139] attempted to enhance the high temperature oxidation resistance (above 

873 K) of 2·25Cr–1Mo ferritic steel by laser surface alloying (LSA) with co-deposited Cr 

using a 6 kW continuous wave CO 2 laser. The main process variables chosen for optimizing 

the LSA routine were laser power (from 2 to 4 kW), scan speed of the sample-stage (150 

to 400 mm/min) and powder feed rate (16 to 20 mg/s). Isothermal oxidation studies in air 

by thermogravimetric analysis at 973 and 1073 K for up to 150 h revealed that LSA had


Figure 18. Kinetics of isothermal oxidation of 

2·25Cr–1Mo ferritic steel laser surface alloyed 

with Cr and exposed to (a) 973 and (b) 1073 K 

in air, respectively [139]. 

significantly enhanced the oxidation resistance of ferritic steel during exposure to 973 and 

1073 K (figures 18a,b). Post oxidation microstructural analysis suggests that an adherent and 

continuous Cr 2 O 3 layer is responsible for the improvement in oxidation resistance [139]. 

Similar improvement in oxidation resistance has been obtained by LSR of bond (Ni–Cr– 

Al–Y) and top (Zr O 2 + Y 2 O 3 ) coated stainless steel [107], LSC of Si-steel with Ni + Cr 

[141], LSM of inconel 600 [142] and LSA of inconel 800 H with Al [108]. 

9.1c Wear resistance by LSE: Attempt to improve wear resistance of steel by LSE seems 

to be more effective than the attempts to enhance the resistance to corrosion and oxidation. 

Cheng et al [110] have added a mixture of WC–Cr 3 C 2 –SiC–TiC–CrB 2 and Cr 2 O 3 to produce 

a metal matrix composite surface on stainless steel UNS-S31603. Following LSM, cavitation 

erosion resistance improved in all cases except for Cr 2 O 3 . Wu & Hong [111] have attributed 

the improvement in hardness and corrosion/wear resistance of stainless steel to the presence 

of amorphous phase in the Zr-rich surface (with 7·8–14·5 at. % Zr) following LSA of 

stainless steel with Zr nano-particles. Roy & Manna [112] have demonstrated that laser surface 

hardening (LSH), instead of LSA or LSM is more effective in enhancing hardness and 

wear resistance of unalloyed austempered ductile iron (ADI). Figures 19 a,b show the typical 

martensitic microstructure developed by LSH and significant improvement in adhesive wear 

of laser hardened vis-à-vis as-received and laser surface melted ADI samples, respectively. 

Adhesive wear of austempered ductile iron consists of three distinct stages: the initial rapid, 

subsequent steady state and final accelerated (abrasive) wear. The improvement in wear resistance 

following LSH is attributed to the martensitic surface with residual compressive stress 

[112].


Figure 19. (a) A predominantly 

martensitic microstructure 

of the laser hardened sample 

surface, and (b) kinetics of wear 

in terms of vertical displacement 

of the pin-head as a function 

of sliding distance during adhesive 

wear testing of austempered 

ductile iron (ADI) subjected to 

LSM and LSH with a pin-ondisc 

machine in dry condition at 

5 kg load. For details, please see 

[112,191,202]. 

Agarwal & Dahotre [113] have reported a substantial improvement in resistance to adhesive/abrasive 

wear of steel following LSA with TiB 2 . The surface composite layer containing 

about 69 vol. % TiB 2 particles recorded an elastic modulus of 477·3 GPa. Similar improvement 

in wear resistance of mild steel was reported by Tondu et al [114] due to formation of 

a FeCr + TiC composite coating formed by laser assisted self propagating high temperature 

synthesis. Earlier, Zhukov et al [120] achieved a similar carbide dispersed composite coating 

on steel by laser assisted self-propagating high temperature synthesis. 

Instead of composite coating, Pelletier et al [115] applied a Fe–Mn–C hadfield steel coating 

by LSC on steel and achieved significant improvement in hardness (over 800VHN) elastic 

modulus (210 GPa) and yield strength (1200 MPa) mostly due to deformation induced twinning 

transformation. A signification mitigation of subsurface crack propagation in pearlitic 

rail steel was achieved following LSM without any change in surface chemistry [116]. Similar 

improvement in wear and friction properties of En 31 steel was obtained by LSH without 

melting or any change in surface chemistry [117]. However, the methods that involve change 

in surface composition seem more popular or effective in enhancing wear resistance of ferrous 

alloys. For instance, LSA of A7 tool steel with Ti was reported to reduce the dry sliding 

friction coefficient due to the formation of a low-friction transfer film [118]. 

Similarly, Stellite coating by LSC is a standard method of reducing wear loss of ferrous 

substrate [119]. Besides providing with a harder and tougher surface, stellite coating develops


a tougher oxide scales and reduces wear under severe condition. Improvement in hardness 

and wear resistance has been achieved by LSC of martensitic steel with Ni alloy [140] and 

LSA of ferritic steel with Cr 3 C 2 + SiC [143]. Several other examples are cited in table 5. 

9.2 LSE of non-ferrous alloys 

Surface dependent degradation by wear, oxidation and corrosion is as much a problem in 

nonferrous metals and alloys as that in ferrous alloys. Thus LSE in widely applied to Al, 

Ti, Cu, Mg and other important nonferrous metals and alloys to extend the service life of 

components subjected to severe conditions of wear, oxidation and corrosion. In this section, 

we will briefly review the scope and current status of understanding regarding the application 

of LSE to enhance surface dependent properties of non-ferrous alloys. For ready reference, 

table 5 summarizes the major work done in the recent past in this area [92–150]. 

Several attempts have been made to improve corrosion resistance ofAl-based alloys by LSM 

[121,125], LSA [122] and LSC/LSR [123,124]. While grain refinement and microstructural 

homogeneity are responsible for better corrosion resistance in LSM dispersion of intermetallic 

and/or amorphous phases are considered crucial for corrosion resistance following LSC/LSR. 

Katipelli & Dahotre [126] have achieved considerable improvement in oxidation resistance of 

6061 Al alloy by LSA with Al+TiC. On the other hand, improved scale adherence is believed 

responsible for enhanced oxidation resistance of Al–Cu–Li and Al–Ga–As following LSM 

[127,128]. 

Improvement in wear resistance of Al alloys seems to necessitate addition of ceramic or 

intermetallic particles by LSC or LSA [129–132]. Usually, the alloyed or cladded layer possesses 

a high hardness and consists of intermetallic or amorphous phases [131,132]. Schnick 

et al [129] induced residual compressive stress by laser surface shocking and achieved a significant 

improvement in wear resistance in HVOF spray coated SiC on Al. 

Manna & Dutta Majumdar [134] attempted to enhance the wear and erosion resistance of 

Cu by laser surface alloying with Cr (electrodeposited with 10 and 20 µm thickness, t z ). Total 

Cr content (X Cr ), Cr in the form of precipitates (f Cr ) and Cr in solid solution with Cu (Cu Cr ) 

were determined by energy dispersive spectrometry, optical microscope and X-ray diffraction 

technique respectively. LSA extended the solid solubility of Cr in Cu as high as 4·5 at. % 

as compared to 1 at. % under equilibrium condition. Figure 20a expresses the variation of 

X Cr , Cu Cr , and f Cr as a function of E for t z = 20 µm. The higher the E, the smaller the 

X Cr , Cu Cr , and f Cr in the alloyed zone. The microhardness of the alloyed zone was found to 

improve significantly (as high as 225 VHN). Since hardness is related to Cr present in solid 

solution and dispersed as precipitates in the matrix, the variation of average microhardness of 

the alloyed zone (Hv 

av) 

as a function of the X Cr, Cu Cr , and w Cr shows that hardness increases 

for all these microstructural factors, especially for Cu Cr (figure 20b). f Cr was converted to 

the corresponding weight fraction (w Cr ) by the simple relation: w Cr = X Cr − Cu Cr . Thus, 

suitable LSA parameters must be chosen to obtain the desired hardness. 

Figure 21a shows the variation of scratch depth (z sc ) as a function of load (L) for pure Cu 

as well as laser surface alloyed Cu with Cr [Cu(Cr)] subjected to scratching with an oscillating 

steel ball in a computer controlled scratch tester. It is evident that the rate of increase of z sc 

with both load and number of scratches is much higher in pure Cu than that in Cu(Cr). Figure 

21b compares the kinetics of material loss (m) of Cu(Cr) lased with 1590 MW/m 2 and 0·08 s 

power and interaction time, respectively (t z = 20 µm) with that of Cu as a function of time (t) 

under an accelerated erosive wear condition conducted both at room and high temperature in 

a slurry bath containing 20 wt. % sand. In addition to reducing the magnitude of m by over 

an order of magnitude, LSA has significantly decreased the kinetics of erosion loss in Cu(Cr)


Figure 20. (a) Variation of Crcontent 

(X Cr ), dissolved Cr in 

solid solution (Cu Cr ) and volume 

fraction of Cr-precipitates (f Cr ) 

as a function of energy density 

(E), and (b) variation of Hv 

av as a 

function of X Cr , Cu Cr and w Cr (= 

X Cr − Cu Cr ). For details, please 

see [134,195]. 

than that in Cu under comparable conditions. Though the extent of material loss increases with 

an increase in T for both pure Cu and Cu(Cr), the rate of erosion loss for Cu(Cr) is negligible 

as compared to a substantial change in m with T for pure Cu, especially beyond 370 K. 

Uniform and refined surface microstructure by LSM seems quite effective in improving 

corrosion resistance in AZ91D/AM60B [136], AZ91 [137], Mg–ZK60 [138] and other commercial 

Mg-based alloys. Recently, Wang & Wue [135] have achieved significant improvement 

in corrosion resistance of SiC dispersed Mg by LSC with Al–12Si alloy layer. 

Laser surface alloying of Ti with Si, Al and Si + Al (with a ratio of 3:1 and 1:3 respectively) 

was conducted to improve the wear and high temperature oxidation resistance of Ti by 

LSA [145,146]. Figure 22a reveals a typical hyper-eutectic microstructure on the top surface 

of the alloyed zone in Ti with Si consisting of faceted Ti 5 Si 3 uniformly distributed in a twophase 

eutectic aggregate of α-Ti and Ti 5 Si 3 [146]. The high volume fraction of the primary 

phase and degree of fineness of the eutectic products signify complete dissolution and uniform 

intermixing of Si in the alloyed zone, and a rapid quenching experienced by the latter, 

respectively. Subsequent oxidation studies conducted at 873–1023 K showed that LSA of Ti 

with Si and Si + Al significantly improved the isothermal oxidation resistance (figure 22b). 

In addition to oxidation, the effect of LSA of Ti with Si or Si + Al on wear resistance was 

also studied. Figure 22c shows the variation of depth of scratching (z w ) with load (L) due to 

scratching of pure Ti, Ti(Si), Ti(Al) and Ti(3Si + Al) with a hardened steel ball. It may be 

noted that z w varies linearly with L for all the cases. The effect of L on z sc is more prominent 

at higher number of scratching (n sc ≥ 1000) than that at a lower value of the same (= 25). 

Under comparable conditions of scratching, Ti undergoes the most rapid wear loss followed


Figure 21. (a) Variation of scratch 

depth (z sc ) with load (L) for different 

numbers of oscillations (n sc ) for 

pure Cu and laser alloyed Cu(Cr), and 

(b) comparison between of material 

loss per unit area (m) due to erosion 

of Cu(Cr) and Cu as a function 

of time (t) at different temperature (T ) 

due to erosion in flowing SiO 2 dispersed 

oil medium. For details please 

see [134,195]. 

by that in Ti(Al), Ti(3Si + Al) and Ti(Si). Ti(Si) undergoes the minimum wear loss. The 

improved wear resistance of laser surface alloyed Ti with Si was attributed to the formation 

of a hard Ti 5 Si 3 precipitates in the alloyed zone [145]. 

Manna et al [199–201] made a novel attempt to develop the material for neural stimulation 

electrode by LSA of Ti with Ir that can mimic the normal spatio-temporal pattern of neuronal 

activation by reversible charge transfer. The usual electrode made of iridium is expensive, 

brittle and not amenable to miniaturization by plastic deformation. On the other hand, titanium 

is cheaper, bio compatible and amenable to drawing/etching. Figure 23a shows the indigenous 

set-up used for fabricating the electrode by LSA. Intelligent combination of laser parameters, 

powder composition and post LSA etching was used to develop the desired microstructure 

(figure 23b). Though charge density could not be measured due to exceedingly uneven surface 

intentionally developed by special etching (meant for increasing the surface area), the total 

charge was comparable to that of pure iridium in appropriate solution [201]. 

9.3 Mathematical modelling on correlation between microstructure and LSE 

Studies on mathematical modelling of LSE mostly focus on predicting the thermal profile 

and composition in the laser irradiated volume. While the knowledge on thermal history and 

solute distribution is essential for predicting the properties, attempts have seldom been made 

to correlate the microstructure and property of the laser treated zone with the LSE parameters. 

Manna & Dutta Majumdar [198] developed a one dimensional heat transfer model based on


(a) 

10 µm 

Figure 22. (a) Scanning electron 

micrograph of the top surface of 

laser surface alloyed Ti with Si, and 

(b) kinetics of oxidation expressed 

as total weight gain per unit area 

(m) as a function of time (t), and 

(c) kinetics of wear in scratch test 

expressed as depth of scratch (z w ) as 

a function of load (L) for Ti, Ti(Si), 

Ti(Al) and Ti(3Si + Al). For details 

about LSA, oxidation and wear test, 

please see [145,194]. 

explicit finite difference method to predict the thermal history (i.e. temperature profile, thermal 

gradient, cooling rate and solid–liquid interface velocity) and hence, the microstructure of 

the alloyed zone developed by laser surface alloying. Figure 24a shows the temperature (T ) 

profile as a function of time (t) during LSA of AISI 304 stainless steel with 100 µm thick 

pre-deposited Mo using a CW–CO 2 laser delivering 1800 MW/m 2 power irradiation with a


(b) 

Tip 

(a) 

Ti 


Ti + Ir powder 

Tilr 

Ti 

1 µm 

SEM 

Figure 23. (a) Schematic diagram showing the set-up used for LSA of Ti with Ir (or Ti + Ir) by 

CO 2 laser pulse for developing neural stimulation electrode, and (b) microstructure of the alloyed zone 

showing a 3 fold increase in surface area following special etching [199,200]. 

25 ms interaction time. The calculation considered the effect of intermixing between the bimetallic 

layer after melting and temperature dependence of concerned material properties. 

The corresponding thermal quenching rate has been shown in figure 24b. The model allows 

calculation of other thermal parameters like thermal gradient, solidification velocity, etc. that 

are useful in predicting the microstructure [204]. Similar model could be extended to twodimensional 

radially symmetric heat transfer condition [205]. 

Roy & Manna [191] have used a simple analytical approach, based on the treatment of 

Ashby & Easterling [188], to predict the thermal profile in LSH of austempered ductile iron 

and explain the phenomenon of partial or complete ‘localized melting’ of graphite nodules 

without liquefaction of the surrounding ferritic matrix. This phenomenon was first reported by 

Fiorletta et al [206]. Figure 25a reveals that a graphite nodule has undergone partial melting 

during LSH of austempered ductile iron with 650W power and 60 mm/s scan speed. Note 

that melting initiates at the graphite-matrix interface and assumes only a part of the nodule 

leaving the core unaffected. The extent or width of this incipient fusion primarily depends on 

the thermal cycle and concomitant carbon diffusion profile from the nodule into the matrix. 

The mathematical model to predict the thermal and composition profile, and hence, the width 

of annular molten region by Roy & Manna [191] could establish a fair-correlation between 

the melt width (y m ) and laser parameters used for LSH. Figure 25b shows the variation of y m 

as a function of depth (z) from the surface predicted for two predetermined conditions of laser 

surface hardening. It is evident that y m decreases as z increases bearing a linear relationship 

between them. The experimental data (open symbols) are fairly close to the predicted trend 

of the model, particularly for higher laser power. 


Laser surface engineering encompasses several applications that are mainly related to enhancing 

one of the surface dependent properties like hardness, friction, fatigue and resistance to 

wear, corrosion, etc. There are several books [1–4,8–10,19] and review articles [18,20,180– 

184] exclusively dealing with this subject. The present section thus made an attempt mainly


Figure 24. (a) Temperature (T ) distribution as a function of time (t) for SS(Mo) at different levels 

of depth (z), and (b) variation of corresponding cooling rate (dT/dt) C as a function of time (t) at 

different z during LSA of AISI 304 stainless steel with Mo. For details, please see [198,204]. 

to review the recent developments and highlight the important issues concerning them. An 

important aspect purposely avoided in this contribution, obviously to avoid exceeding the 

prescribed length, concerns mathematical modelling of LSE. For this, readers may see references 

[5–6,185–186] and obtain most of the necessary data on material properties from the 

references [189–190]. The current trend appears to suggest that the excimer and diode lasers 

with shorter wavelengths may soon substitute CO 2 even YAG lasers for LSE applications. 

Sustained efforts will then be needed to establish the microstructural evolution and scope of 

improvement following LSE with those new lasers. 

Apart from structural applications, LSE could be equally useful in enhancing functional 

properties like magnetism, emission/absorption characteristics, sensors, microelectronic


Melt width y m (µm) 

100 (b) 

80 

60 

40 

20 

P = 700 W, 

V = 60 mm/s 

P = 1000 W, 

V = 20 mm/s 

80 100 120 140 

Depth from the top surface z (µm) 

Figure 25. (a) Partial melting of a graphite nodule embedded in the ferritic/martensitic matrix following 

LSH with 650W power and 60 mm/s scan speed. Note that melting initiated at the graphite-matrix 

interface and consumed nearly half the nodule from the circumference, and (b) variation of predicted 

melts widths (y m ) around graphite nodules as a function of depth (z) from the surface. Open and solid 

symbols represent experimental and predicted data, respectively [191]. 

devices and several other applications requiring monolithic or functionally/ compositionally 

graded microstructures confined to smaller dimensions. The LSE processes described in 

this section are equally applicable to such functional application, provided the correct laser 

parameters (wavelength, mode, energy etc.) are selected for the given materials (semiconductor, 

polymer etc.). 

The major outstanding issues concerning LSE are: (a) utilization of shorter wavelengths 

for more precise surface engineering, (b) developing compositionally and functionally graded 

microstructure, (c) elimination or minimization of structural/coefficient mismatch and combining 

dissimilar materials. In the next section, we shall introduce a major challenge concerning 

LSE that might prove a technological breakthrough if it is possible to convert the 

scientific feasibility into commercial reality. 

10. Laser surface vitrification – A challenge 

It is known that environmental degradation by corrosion and oxidation alone accounts for more 

than 30% discard and loss of engineering components of all dimensions from bridges/railwaycarriages 

to needles/bolts made of metals and alloys. While replacing a bolt/structural member 

in a bridge or painting a carriage is neither too expensive nor difficult, an early malfunction 

of a heart valve, orthopedic implant or aeroturbine engine component may pose threat to the 

human life, cause tremendous trauma and lead to catastrophic failure respectively. 

One or higher dimensional structural defects in a polycrystalline aggregate like dislocations, 

homo/heterophase boundaries, voids and inclusions constitute the higher energy hence 

preferred nucleation sites for materials degradation by oxidation and corrosion. On the other 

hand, an amorphous state is devoid of both crystalline anisotropy and intercrystalline defects, 

and thus, does not provide the nucleation sites and short-circuiting diffusion path for such 

environmental degradation. However, an amorphous or glassy material, more often than not, 

is brittle and not amenable to material processing/shaping to develop a large component 

of complex geometry. Thus, a crystalline solid with an amorphous surface or over-layer is


obviously superior to a usual crystalline engineering component for applications in hostile 

environment where aqueous corrosion and high temperature oxidation are anticipated. 

Since the maiden success of Klement et al [151] in 1960 in developing Au–Si metallic glass 

by rapid quenching from the liquid stage, considerable research efforts have been devoted in 

the past few decades to form metallic glasses with other metals/alloys. It is now generally 

accepted that all liquid metals/alloys may be transformed to the glassy state provided crystallization 

is overridden during solidification [152]. The cooling rate required to retain the 

glassy state, which is the maximum for monoatomic pure metals [153], is a strong function 

of the melt composition. Rapid solidification processing (RSP) including splat cooling is by 

now an established technology in producing metallic glasses. However, limitation in the maximum 

cooling rate achievable (< 10 5 –10 8 K/s), stringency of melt composition amenable 

to amorphization and constraints of physical dimensions of the amorphous ribbons/products 

have severely restricted the scope of RSP to produce metallic glasses. 

In this regard, the landmark discovery of Lin & Spaepen [154] to form metallic glasses 

from compositionally modulated Fe–B thin film sandwich using pico-second pulse laser irradiation 

pioneered a unique possibility of laser assisted surface vitrification or amorphization. 

Suppression of partition-less crystallization was attributed to both the ultra-high quenching 

rate achieved (10 10 − 10 13 K/s) and preference to collision-limited transformation over the 

interface or diffusion-limited mechanism due to the slow kinetics of the latter. The results 

assumed particular significance because amorphization of only the surface of a crystalline 

material was earlier believed not feasible due to the scope of epitaxial growth of partitionless 

or partition-involved crystallization from the underlying solid substrate. 

Table 6 presents an exhaustive list of systems known to have undergone laser assisted surface 

amorphization. Snezhnoi et al [155] were the first to report the presence of an amorphous 

phase on the laser remelted chilled (ledeburitic) cast iron surface showing an average hardness 

of 1200 H V . Subsequently, Bergmann & Mordike [156,157] were successful in developing 

dispersion of Fe–B based amorphous phases in laser surface quenched cast tool steel, Cr-steel 

Table 6. Summary of work done on laser surface vitrification (LSV). 

Year Substrate/deposit Laser Ref. 

1980 Chilled cast iron Nd: Glass pulsed [155] 

1980 Cast tool steel/Fe–B (sprayed) CW–CO 2 [156] 

1981 Fe–2C–12Cr/Fe–B Nb-alloy CW–CO 2 [157] 

1981 Fe–C/Si–P–B (ternary/quaternary) TEA–CO 2 Pulsed [158] 

1982 Fe–Fe 3 B (modulated thin film) Nd:YAG Pulsed [154] 

1984 Fe-4 at.% B Nd:YAG Pulsed [160] 

1984 Mo/Ni (30–60 at%) Mo/Co (45 at%) Co/Nb (40 at%) Nd:YAG mode locked [161] 

1984 Ni–Nb thin film Nd:YAG [160] 

1984 Zr/Cu Nd:YAGQ-switched [162] 

1984 Au–Ti, Co–Ti, Cr–Ti, Zr–Ti Pulsed [159] 

1984 Pd–6Cu–16Si CW–CO 2 [166] 

1986 Fe–10Si–15B Pulsed CO 2 [168] 

1987 Fe–10Cr–5Mo/12–14 P, C CW–CO 2 [169] 

1985 Pure Ga KrF excimer [171] 

1987 Mild steel/Ni–Cr–16P–4B CW–CO 2 [169] 

1989 Ni, Cu(Ni), Ti(Ni)/Pd–25Rh–10P–9Si CW–CO 2 [170] 

1988 Nb/Ni–Pt–Pd–Rh CW–CO 2 [165] 

1988 Fe–Cr–P–C–Si CW–CO 2 [174] 

1990 Review-paper [163]


and Nb-alloys. Borodina [158] met with similar success of amorphizing the surface of several 

ternary and quaternary Fe-metalloid (B/C/Si) compositions up to 7–20 µm depth. Affolter 

& von Allmen [159] attempted to establish the criteria for surface amorphization by laser 

irradiation with Au–Ti, Co–Ti, Cr–Ti and Zr–Ti binary systems. While surface amorphization 

was achieved in Au–Ti, Co–Ti and Cr–Ti systems, failure to convert Zr–Ti couple into the 

glassy state was attributed to the scope of easy partitionless crystallization enabled by complete 

mutual solubility both in liquid and solid state in Zr–Ti. 

Lin & Spaepen [160] extended their earlier approach to produce amorphous layers in Ni– 

Nb [160], and Mo–Ni, Mo–Co and Co–Nb couple [161]. A systematic study on the genesis 

of surface amorphization appears to suggest that impurity stabilization of short-range order 

in the liquid state is primarily responsible to prevent diffusion controlled or partitionless 

crystallization during laser quenching. For instance, the formation of the metallic glass in Cu– 

Zr by den Broeder et al [162] is a typical case of impurity stabilized surface amorphization. 

Hashimoto et al [163] have reviewed a number of experimental studies [164–170] by their 

own group to emphasize that laser assisted surface amorphization is a feasible method of 

improving corrosion resistance of bulk crystalline alloys. 

Thus, a sizable number of experimental studies seem to evidence that surface amorphization 

of laser irradiation is feasible under a chosen composition range and optimum LSE condition. 

In this regard, the extreme heating/cooling rates, thermal gradient and resolidification-velocity 

may aid the collision controlled partitionless amorphization of the melt. However, Kear et al 

[172] and Bergmann & Mordike [156,157] pointed out that suppression of epitaxial nucleation 

and prevention of growth of crystallites from the underlying solid substrate must be insured 

to achieve this laser assisted surface amorphization. 

Spaepen [173] has worked out the thermodynamic and kinetic criteria for surface amorphization 

for picosecond pulse laser irradiation of metals. It is suggested that partitionless 

crystallization may occur at an interface velocity (v i ) of 100 m/s so long as the isotherm 

velocity (v T ) coincides with v i . When v i


Figure 26. A schematic binary phase diagram 

showing the plunging T o -curves for the terminal 

solid solutions. The shaded region represents 

the composition range (below T g )in 

which partition-less solidification may lead to 

amorphization or vitrification. 

where k is the thermal conductivity, T is the thermal gradient, V m is the molar volume, and 

H c is the latent heat of crystallization. 

The solidification is ‘heat flow limited’ when v i,th ≪ nz and T i ≃ T m . This is the case for 

usual solidification and crystal growth. However, v i,th ≫ nz and T m >T i in LSE such that the 

growth is ‘interface-limited’ leading to the formation of metastable phases and glassy state. 

Furthermore, depending on the magnitude of the product nz, solidification may either belong 

to collision controlled (when n ≃ thermal vibration frequency) or diffusion controlled (when 

n = diffusion jump frequency) regime. While the former process (applicable to pure metals, 

dilute alloys and intermetallic compounds) is difficult (or impossible) to suppress, the latter 

mechanism may easily be overridden if T is large or T i ≪ T m and T i


of crystalline phases due to poor atomic mobility and high viscosity. However, attaining this 

glassy state will crucially depend upon the melt composition, heat and mass transfer condition 

and suppression of epitaxial nucleation that primarily depends on the ability of maintain 

the condition: T i


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